Self-executing code on a blockchain that automatically enforces the terms of an agreement when conditions are met.
A smart contract is a program stored on a blockchain that automatically executes predefined logic when specific conditions are met — eliminating the need for intermediaries or trust between parties. Coined by cryptographer Nick Szabo in 1994 and first broadly implemented on Ethereum, smart contracts power DeFi (automatically liquidating undercollateralized loans), DEXs (executing token swaps), DAOs (enforcing governance votes), NFTs (managing ownership and royalties), and much more. Smart contracts are immutable once deployed — the code cannot be changed, which provides certainty but also means bugs cannot be patched (unless the contract has upgrade mechanisms). This immutability is why smart contract audits are crucial: a single vulnerability can lead to millions or billions in losses. Solidity is the dominant smart contract language (Ethereum), while other chains use Rust (Solana), Move (Sui, Aptos), and Cairo (Starknet).
Smart contracts are self-executing programs stored on the blockchain that automatically enforce agreed-upon conditions — 'if this, then that' logic with no intermediary required. They power DeFi lending (automatically liquidating undercollateralized loans), DEX trading (executing swaps through AMM formulas), and NFT mechanics (enforcing royalties on secondary sales). Once deployed, smart contracts execute exactly as written — they cannot be altered by their creators or anyone else (unless designed with upgrade mechanisms). This immutability is both the greatest strength (trustless, predictable execution) and weakness (bugs in deployed contracts cannot be fixed). The total value locked in smart contracts across all chains exceeds $100 billion, representing enormous trust in code. Smart contract auditing has become a critical industry — firms like Trail of Bits, OpenZeppelin, and Certora review code before deployment, but audits are not guarantees of safety. Major exploits continue to occur despite audits.
Aave's smart contracts manage $10B+ in crypto loans autonomously — automatically calculating interest rates, holding collateral, and liquidating positions when collateral ratios drop below thresholds, all without any human intervention.
Standard smart contracts cannot be changed — their code is immutable once deployed. However, many protocols use 'upgradeable proxy' patterns that allow logic changes through governance. This upgradeability introduces trust requirements: someone (usually a multisig or DAO) has the power to change the code. Check if a protocol's contracts are upgradeable before depositing significant funds.