Key Takeaways
Quick answer
A smart contract is code that lives on a blockchain and executes automatically, exactly as programmed, whenever someone calls it. Ethereum was designed to run them. They power tokens, exchanges and lending apps, but ethereum.org warns that interactions are irreversible and buggy code is hard to fix.
- A smart contract is a program on a blockchain that runs exactly as written.
- Anyone can deploy one, so nobody checks a contract before you use it.
- Interactions are irreversible, and flawed code has led to large thefts.
What is a smart contract in plain English?
ethereum.org keeps it simple: “A ‘smart contract’ is simply a program that runs on the Ethereum blockchain.” It holds code and data at its own address on the blockchain. NIST, the US standards body, describes smart contracts as software deployed on a blockchain and run by the computers that operate it.
ethereum.org likens it to a vending machine: the right money plus a selection reliably produces the snack, with nobody behind the counter. Despite the name, it is software, not a signed legal agreement.
What are smart contracts used for?
On Ethereum they sit behind tokens, swaps on a DEX and pool-based lending, which ethereum.org describes as lenders adding funds to a pool that borrowers draw from. Every call costs a gas fee. Contracts also cannot fetch real-world data by themselves; services called oracles feed it in.
Who can create a smart contract?
Anyone. ethereum.org states: “Anyone can write a smart contract and deploy it to the network.” That openness is the point, but it also means nobody vets a contract before you use it. Scam tokens and rug pulls rely on exactly this.
What are the risks of smart contracts?
ethereum.org warns: “Smart contracts cannot be deleted by default, and interactions with them are irreversible.” Its security guide adds that deployed code usually cannot be changed to patch flaws, and stolen assets are mostly unrecoverable. NIST records that in 2016 flaws in a contract called the DAO let an attacker extract ether, a theft of $50 million.
Frequently asked questions
Can a smart contract be changed after it goes live?
Assume not. ethereum.org says deployed code usually cannot be changed, so the code you interact with is normally the code that keeps running.
Does a smart contract count as a legal contract?
ethereum.org and NIST describe programs, not legal agreements. Whether an arrangement is enforceable in court depends on the law, not on the code.
Article Sources
4 sources
Blockhorizon checks every figure, date and quotation against primary sources: regulators, statistics bodies and original technical documents. Read our editorial policy.
- ethereum.org, Introduction to smart contracts (updated Feb 2026) — ethereum.org (accessed 2026-10-02)
- ethereum.org, Smart contract security (updated Feb 2026) — ethereum.org (accessed 2026-10-02)
- NIST IR 8202, Blockchain Technology Overview (2018) — nvlpubs.nist.gov (accessed 2026-10-02)
- ethereum.org, What is DeFi? — ethereum.org (accessed 2026-10-02)