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An Ethereum smart contract is a program deployed at an address on the Ethereum blockchain. It contains code and data, and the Ethereum Virtual Machine (EVM) runs its code when an interaction requests it. State-changing execution is recorded on Ethereum and uses computational resources measured in gas.
What “smart contract” means on Ethereum
Ethereum.org defines a smart contract as “simply a program that runs on the Ethereum blockchain,” made up of functions and state stored at a specific blockchain address. In plain terms, the code describes what the contract can do, while its state holds the data it uses or updates.
The word “contract” here means executable software, not necessarily a legal agreement. A separate legal contract may exist, but the technical term alone does not establish one. Nor is a smart contract a self-starting robot: its code runs when an interaction triggers it and the required conditions are met.
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Where the code lives and who controls it
Deployment creates a contract account at an Ethereum address. Unlike an externally owned account (EOA), which is controlled by private keys and can initiate transactions, a contract account is controlled by its code. Developers commonly write contracts in languages such as Solidity or Vyper, then compile them into bytecode that the EVM can execute.
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A deployment is a transaction that creates the contract account. Later interactions can call its functions. When a call changes data or other Ethereum state, that change takes effect as part of the transaction’s execution and is committed to the network.
Ethereum accounts · Smart contract languages
How a smart contract runs
The EVM is Ethereum’s execution environment and is described as a distributed state machine. Ethereum nodes execute the same contract instructions deterministically: given the same relevant state and inputs, the code follows the same rules. Users or applications request execution through a transaction or another supported interaction; the EVM does not independently decide to run a contract.
A vending machine is a useful analogy: a selection and payment trigger a predictable response according to its programmed rules. The analogy has limits. A smart contract does not inherently know about events in the physical world, and it acts only when execution is triggered.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors- Deploy: a transaction creates the contract account and places compiled bytecode on Ethereum.
- Call: a user, application, or another contract requests one of its functions.
- Execute: the EVM runs the relevant code against the current state.
- Commit changes: if the transaction succeeds and changes state, the resulting update is recorded on Ethereum.
Technical introduction to Ethereum · Ethereum Virtual Machine (EVM)
Transactions, reads, and gas
Gas measures the computational resources consumed by Ethereum execution. Deploying a contract and making a state-changing call normally requires a transaction and gas. A read-only call that does not modify state does not cost gas according to Ethereum.org’s interaction guide. Applications often use such reads to display contract data without submitting a state-changing transaction.
The usual flow involves an EOA signing a transaction, but it is not the only possible account and fee arrangement. Account abstraction can allow a smart-contract account to initiate writes, and a paymaster can cover gas. The interaction model therefore depends on how the application and account are set up.
Interacting with smart contracts
What smart contracts can—and cannot—do
- Apply programmed rules: code can transfer assets or update contract data when a call meets its conditions. It follows the rules it was given, including any errors or unsafe logic.
- Work with other on-chain software: contracts are public and can be inspected or called by applications and other contracts, subject to each application’s design.
- Use off-chain facts only through an input mechanism: a contract cannot directly fetch information from the internet or observe real-world events. An oracle can make external data available on-chain.
- Make consequential, hard-to-reverse changes: successful state-changing transactions update Ethereum’s state. Users should not assume that a mistaken transaction can simply be undone.
Technical introduction to dapps
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Are Ethereum smart contracts immutable?
Deployed contract code is generally immutable by default, and Ethereum’s introductory guide says contracts cannot be deleted by default. That does not mean every application’s behavior is permanently fixed. Some systems use upgrade patterns, such as separating business logic from the contract users interact with, so that calls can be routed to different logic.
Whether an application can be upgraded depends on its design and the controls around those upgrades. An upgrade pattern changes which logic users reach; it does not mean the original deployed bytecode itself has been rewritten.
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Practical limits and security
Deterministic execution does not guarantee that a contract is safe. The EVM runs the instructions as written, so bugs and poorly chosen rules can produce unintended outcomes. Ethereum.org’s dapp guidance recommends careful design and thorough testing.
Ethereum.org’s smart-contract introduction states a maximum contract size of 24 KB. This is a protocol detail reported by that guide, whose publication year is not stated; check the current documentation when the limit matters for a specific deployment.
Anatomy of smart contracts · Technical introduction to dapps
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