Smart Contract definition
A smart contract is a program stored on a blockchain that runs automatically when predefined conditions are met. It holds and moves digital assets, enforces rules and records outcomes without an intermediary, and once deployed its code and execution are visible and verifiable on the network. Smart contracts power tokens, DeFi protocols, NFTs and DAOs.
How do smart contracts work?
Developers write a smart contract in a language such as Solidity or Vyper for Ethereum-compatible chains, or Rust for Solana, compile it, and deploy it to the blockchain, where it receives an address. Users and other contracts interact with it by sending transactions that call its functions. Every node executes the same code and reaches the same result, so the outcome, such as a token transfer or a loan liquidation, is enforced by the network itself.
Execution costs fees, called gas on Ethereum, which pay validators and prevent infinite loops from clogging the network. Contracts cannot fetch outside data directly, so they rely on oracles, such as Chainlink, to bring in prices, weather or other real-world information when needed.
Examples of smart contracts
Smart contracts are the building blocks of most blockchain applications. Many follow published standards, such as ERC-20 for fungible tokens and ERC-721 for NFTs, so wallets, exchanges and other applications can work with them automatically. Typical examples are listed below.
- Token contracts that create and transfer cryptocurrencies or stablecoins.
- Decentralized exchanges that swap tokens using liquidity pools.
- Lending protocols that issue loans against collateral and liquidate automatically.
- NFT contracts recording ownership of digital items and paying creator royalties.
- Escrow and payment release when delivery conditions are confirmed.
- DAO governance contracts that execute approved proposals.
Smart contract risks and vulnerabilities
Because deployed contracts often control real money and cannot easily be changed, bugs are expensive and public. Well-known vulnerability classes include reentrancy, where an external call re-enters the contract before its state updates, as exploited in the 2016 attack on The DAO; access control mistakes; integer and rounding errors; oracle manipulation using flash loans; and unsafe upgrade patterns. Attackers actively scan new contracts for these weaknesses.
Upgradeable proxy patterns allow fixes but add their own risks and trust assumptions, since whoever controls the upgrade key can change behavior after deployment. Many teams therefore add timelocks so users can exit before an upgrade takes effect, and publish clear documentation of who holds which keys.
How to build smart contracts safely
Use audited libraries such as OpenZeppelin Contracts rather than writing standard components from scratch. Write extensive tests, including fuzzing and invariant testing with tools like Foundry or Hardhat, and run static analyzers such as Slither. Keep contracts small and simple, minimize privileged roles, protect admin keys with multisig wallets and timelocks, and commission independent security audits before mainnet launch. Bug bounty programs and monitoring add protection after deployment.
Are smart contracts legally binding?
A smart contract is code, not automatically a legal contract. Whether an arrangement executed through code is legally enforceable depends on jurisdiction and on whether the elements of a contract exist, such as offer, acceptance and intent. Many projects pair smart contracts with traditional legal agreements. Nexzem's smart contract developers design, test and prepare contracts for audit on Ethereum, EVM chains and Solana.