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A smart contract is a program deployed to a blockchain address. It stores rules and data, and changes the blockchain’s shared state when a user or another contract sends it a valid transaction. On Ethereum, that program runs in the Ethereum Virtual Machine (EVM).
Despite the name, a smart contract is not automatically a legal contract, inherently intelligent, private, or autonomous. It is software that follows predefined logic. Its reliability depends on the code, blockchain, cryptographic keys, external data providers, administrators, and the assumptions built into the application.
The simplest way to understand a smart contract
Think of a smart contract as shared software running on a blockchain rather than on one company’s server. Users and other programs can call its functions, the network’s nodes execute the same deterministic code, and the resulting changes are recorded for others to verify.
A vending machine is a useful—but imperfect—analogy: insert the required payment, satisfy the machine’s programmed condition, and it delivers the selected item. A smart contract similarly checks inputs and conditions before transferring a token, recording a vote, issuing an asset, or performing another programmed action.
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The analogy breaks down in important ways. A contract does not normally wake up and execute by itself. It needs a transaction from a wallet, another contract, an oracle, or an automation service. It cannot independently know whether a physical delivery occurred, whether weather conditions changed, or whether an off-chain promise was honored. And its code may contain bugs or give an administrator significant control.
Ethereum describes smart contracts as public, composable application interfaces: one contract can call another. That allows applications such as exchanges, lending platforms, wallets, marketplaces, and governance systems to be assembled from on-chain components. Learn more from Ethereum’s smart-contract documentation.
How a smart contract works, step by step
1. A developer writes the rules
For Ethereum and many compatible networks, developers commonly use Solidity or Vyper. Other platforms use different languages and execution environments, so an Ethereum example should not be treated as the definition of every smart contract.
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2. The source code is compiled
A compiler converts readable Solidity into EVM bytecode. It also produces an ABI, or application binary interface, describing callable functions, their parameters, and events. Wallets and front ends use the ABI to construct transactions and display results.
During deployment, the blockchain receives creation bytecode. That code runs once and returns the runtime bytecode that is stored at the new contract address; the deployed contract does not simply receive the original human-readable source file. Solidity’s documentation explains compilation and EVM execution.
3. The contract is deployed
Deployment is a blockchain transaction that normally has no recipient address and contains the compiled contract code. The network executes the creation code, assigns an address, stores the resulting runtime code, and establishes the initial state.
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Deployment requires the network’s native asset to pay gas and generally costs more than a simple transfer. Ethereum’s deployment guide describes this process.
4. Someone calls a function
A wallet creates and signs a transaction using a private key. The transaction identifies the contract, encodes the function name and arguments, and offers a gas limit and fee. It is then broadcast to the network.
A user clicking “swap” is one example. A liquidation bot calling a lending function, or an oracle submitting a price update, are others.
5. Nodes execute the transaction
The EVM runs the code against the blockchain’s current state. Validating nodes must be able to reach the same result from the same prior state and transaction input. The contract may:
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- Read or update balances, ownership records, votes, deadlines, or other state.
- Check permissions and conditions.
- Transfer cryptocurrency or tokens.
- Call another contract.
- Emit events and logs.
- Revert if a requirement is not met.
6. The blockchain records the result
If execution succeeds, the chain records the updated state and a transaction receipt containing status information and, often, event logs. Applications watch those logs to show a completed trade, payment, vote, or transfer.
If the transaction reverts, the state changes from that call are undone. The sender may still lose gas already consumed, so a failed transaction is not necessarily free. Running out of gas also causes failure and does not guarantee recovery of gas already spent.
What is inside a smart contract?
| Part | What it does |
|---|---|
| Code | Defines functions, rules, calculations, and permitted actions. |
| State | Persistent on-chain data such as balances, owners, votes, prices, and deadlines. |
| Address | The blockchain location that users and other contracts call. |
| Events and logs | Execution records that wallets, websites, and monitoring systems can read. |
| Access control | Rules limiting sensitive functions to an owner, role, multisig, or governance process. |
| Fallback and receive behavior | Special handling for unmatched calls or incoming native assets. |
| External calls | Interactions with other contracts, enabling composability while importing additional risk. |
Ethereum distinguishes persistent storage from temporary memory. Storage survives transactions but is comparatively expensive to modify, so efficient data layout and minimizing unnecessary writes matter for cost and performance. See Ethereum’s contract anatomy reference.
A concrete example: blockchain escrow
Suppose Alice deposits a digital asset into an escrow contract for a transaction with Bob. A simplified flow could be:
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- The contract records the deposit and the intended recipient.
- An authorized approver, dispute process, or oracle submits a release transaction after the agreed condition is met.
- The contract checks its rules and transfers the asset to Bob.
- If the condition is not satisfied, the call reverts or follows an alternative refund or dispute path.
The contract can verify on-chain facts—such as whether a deposit was made or a deadline passed. It cannot independently know that a physical package arrived. That fact must come from an oracle or an authorized external input, which creates an additional trust and failure point. Ethereum’s oracle guide explains why blockchains need this mechanism.
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A toy release function might look like this:
function release() external {
require(msg.sender == authorizedApprover, "not authorized");
require(locked, "already released");
locked = false;
payable(recipient).transfer(amount);
}This is educational pseudocode, not production-ready financial software. It omits reentrancy protection, pull-payment design, failure handling, dispute resolution, initialization, access-control setup, upgrade policy, and testing. Never deploy an example like this with real funds.
What are gas and smart-contract fees?
Gas measures the computational work and certain resource usage required by a blockchain transaction. The sender pays for that work using the network’s native asset.
In broad terms, the cost is determined by the gas used and the applicable gas price or fee mechanism. Deployment and complex contract calls usually consume more gas than simple transfers. A block’s gas capacity also limits how much computation can be included.
There is no universal “smart-contract fee.” The amount varies with the blockchain, network congestion, transaction complexity, fee market, and whether the application runs on a Layer 2 or another scaling network. A reverted call can still consume gas.
Are smart contracts automatic?
Only conditionally. A smart contract normally requires a transaction trigger. That trigger may come from:
- A user signing a transaction in a wallet.
- Another smart contract calling it.
- A liquidation bot or automation service.
- An oracle submitting updated information.
“Self-executing” usually means that once a triggering transaction satisfies the coded conditions, the network applies the programmed result without a conventional intermediary manually approving each step. It does not mean the contract has a clock, internet connection, or independent intention.
What smart contracts are used for
- Finance: lending, collateral management, exchanges, escrow, derivatives, and conditional payments.
- Tokens and ownership: issuance and transfer of fungible tokens, NFTs, memberships, and digital collectibles.
- Marketplaces and auctions: rules for bids, settlement, ownership transfer, and fees.
- Governance: voting, treasury controls, proposals, and delegated permissions in DAOs.
- Wallet administration: multisignature approval, spending limits, recovery procedures, and account abstraction features.
- Games and digital goods: on-chain items, rewards, and player-to-player transfers.
- Enterprise coordination: shared records and workflows where multiple organizations need a common execution layer.
An NFT can record token ownership and enforce programmed transfer rules, but the token does not automatically confer legal ownership of a physical item or guarantee that a creator will pay royalties off-chain. Real-world assets similarly require custodians, legal agreements, data providers, and enforcement mechanisms beyond the token itself.
What smart contracts cannot do
They cannot directly read the outside world
Independent nodes must execute deterministically. If each node fetched changing web data independently, they could receive different answers and fail to agree. Oracles bring external information on-chain, but they also introduce provider, availability, and data-quality assumptions.
They cannot force real-world compliance
A contract can transfer a blockchain asset. It cannot by itself force someone to ship a product, honor an off-chain promise, or obey a court order.
They cannot correct bad inputs
If an oracle reports a wrong price, the contract may execute perfectly according to incorrect information. A permanent record proves what was submitted, not necessarily that the submission was true.
They are not automatically private
On public chains, code, transactions, balances, and event data may be visible. Addresses are often pseudonymous rather than anonymous and can sometimes be linked through transaction history, exchange records, public metadata, or application behavior.
They are not always immutable
Ordinary deployed code may be difficult or impossible to change. But proxy contracts and upgrade mechanisms can let authorized parties replace the implementation or modify parameters. A system advertised as immutable should be checked for owner keys, pause functions, upgrade authorities, and governance powers.
They do not eliminate every intermediary
Users may still rely on oracle operators, bridge providers, sequencers, RPC providers, wallet software, front ends, auditors, administrators, and multisignature signers. “Decentralized” is a spectrum: ask what is decentralized—validation, custody, governance, data sourcing, transaction ordering, or the user interface.
Smart-contract risks
A contract can be unsafe even when its transactions are visible and its code is verified. Important risks include:
- Reentrancy and unsafe external calls.
- Broken access controls or compromised administrator keys.
- Arithmetic and logic errors. Solidity 0.8.0 and later reject many arithmetic underflow and overflow cases by default, but that does not prevent broader vulnerabilities.
- Oracle manipulation, stale data, outages, or flash-loan-assisted attacks.
- Front-running and transaction-order dependence.
- Denial-of-service conditions and unexpected gas exhaustion.
- Proxy, upgrade, and initialization vulnerabilities.
- Bridge and cross-chain messaging failures.
- Economic attacks that exploit valid rules rather than a coding mistake.
- Malicious token approvals that grant excessive spending permission.
- Compromised wallets or lost private keys.
- Dependencies and libraries with their own bugs or assumptions.
- A compromised front end that displays misleading transaction parameters or sends users to a fraudulent contract.
An audit is evidence of a review within a defined scope and time period—not a guarantee that the code is bug-free, economically sound, or safe after later upgrades. Ethereum’s security guidance covers common vulnerabilities and defensive practices.
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On Ethereum, an externally owned account (EOA) is controlled by a private key. It can sign and initiate transactions. A contract account is controlled by code at a blockchain address. It cannot initiate a transaction by itself, but it can respond to calls and call other contracts.
A wallet is usually an application or device that manages keys and helps a person interact with accounts. A smart-contract wallet—such as a multisignature wallet—uses contract logic for approvals. In an N-of-M arrangement, a specified number of signers must approve an action; for example, three of five signers could be required. This reduces dependence on one key but adds coordination and recovery challenges.
How to build one responsibly
- Define the requirements and threat model. Decide what must be on-chain, who can act, what happens on failure, and which assumptions depend on an oracle or administrator.
- Choose the platform. Compare execution environment, language, finality, fees, privacy, throughput, tooling, composability, and upgrade model.
- Implement minimally. Use a supported compiler and established libraries where suitable. Minimize privileged roles and document every administrator capability.
- Test failure paths. Add unit and integration tests for permissions, edge cases, reverts, unusual token behavior, deadlines, and loss of keys. Use fuzzing and invariant testing for important assumptions.
- Test under realistic conditions. Use a local environment and testnet, simulate congestion and external calls, and inspect gas usage.
- Review and verify. Obtain independent review appropriate to the value and complexity. Verify deployed bytecode against the source code so users can inspect what is actually deployed.
- Deploy with controlled administration. Use a multisig where appropriate, protect upgrade keys, and make pause or emergency procedures explicit.
- Monitor after launch. Watch events, privileged actions, oracle updates, unusual transfers, and failed transactions. Prepare an incident-response plan before value is at risk.
Common development options include Remix for browser-based learning and prototypes, Hardhat or Foundry for local development and testing, and OpenZeppelin Contracts for reusable Solidity components. Hosted RPC providers such as Infura and Alchemy can provide network access without operating a node. Their pricing, limits, supported networks, and availability change, so check the official pages before choosing one.
If a project needs external data or automated triggers, it may consider oracle infrastructure such as Chainlink, while remembering that an oracle adds a trust and availability assumption. For treasury or deployment administration, a multisignature system such as Safe can reduce single-key risk. Neither tool replaces testing, review, operational security, or legal work.
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| Question | Smart contract | Conventional system |
|---|---|---|
| Who executes it? | Blockchain nodes under network consensus. | A company or operator’s servers. |
| Where is state stored? | On-chain, often publicly visible. | Usually in controlled databases. |
| How is access authorized? | Cryptographic signatures and contract permissions. | Accounts, sessions, and operator controls. |
| How easily can rules change? | Often difficult, unless upgrade mechanisms exist. | Usually easier for the operator to update. |
| Cost and speed | Every transaction consumes network resources and may be slower or variable in cost. | Often cheaper and faster for high-volume internal workflows. |
| External data | Requires oracle or relay infrastructure. | Servers can call APIs directly. |
A smart contract may be a good fit when several parties need a shared, tamper-resistant record; the rules are precise and deterministic; digital assets are already on-chain; public verifiability or composability matters; and users can accept fees, latency, keys, and irreversible transactions.
It may be a poor fit when data must remain confidential, rules change frequently, transaction volume is high, a trusted operator already solves the problem efficiently, most inputs come from off-chain sources, or users require easy cancellations, refunds, and conventional customer support.
Are smart contracts legally binding?
“Smart contract” is a technical term, not a universal legal classification. Whether code-based performance forms or enforces an agreement depends on the jurisdiction, the parties, the facts, the governing law, and the surrounding contract structure.
A blockchain transaction may help prove that an action occurred, but it does not by itself resolve identity, authority, fraud, consumer protection, ownership, remedies, or court enforcement. For a real transaction or regulated product, obtain advice from a qualified attorney in the relevant jurisdiction rather than assuming that code replaces a written legal agreement.
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Which blockchains support smart contracts?
Ethereum is one prominent example, but it is not the only platform. Smart-contract systems also include Ethereum-compatible networks, Solana programs, Bitcoin Script-based applications, Cosmos and CosmWasm ecosystems, Polkadot/Substrate environments, Move-based networks such as Sui and Aptos, Starknet/Cairo, Stellar Soroban, and enterprise or permissioned platforms using systems such as DAML.
They differ in programming language, execution model, consensus and finality, fees, throughput, privacy, developer tooling, composability, and governance or upgrade arrangements. Ethereum’s developer tooling catalog lists tools and ecosystems beyond the EVM.
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