Optimistic rollups: what they are and how they work
An optimistic rollup is an Ethereum scaling network that assumes transactions are valid and checks them if challenged. Batches post to Ethereum.
Vahe HakobyanEditor-in-chief
Updated Oct 6, 20263 min readFact-checked
On this page
- Optimistic rollups batch transactions and post them to Ethereum.
- Fraud proofs let anyone challenge a bad batch.
- Withdrawals can wait out a challenge period.
- A sequencer orders transactions and may be run by one company.
- Zk rollups prove validity with cryptography instead of assuming it.
Ethereum processes its own transactions, so activity can slow and fees can rise when demand is high. An optimistic rollup moves most of that activity onto its own network and keeps a record on Ethereum.
How do people use it?
To use an optimistic rollup, you first get assets onto it. Most people bridge from Ethereum, and many bridges lock your tokens there while issuing a matching amount on the rollup. Some exchanges let you withdraw straight to a rollup; either way, you transact there and later withdraw back to Ethereum. Check your wallet is on the right network before you send, because funds sent to the wrong chain can be hard to recover.
How does it work and what can fail?
A rollup takes many transactions, batches them, and posts the batch to Ethereum as compressed data. Ethereum keeps that record, which gives the rollup its security. If someone spots a bad batch, they can submit a fraud proof during a challenge period to show it is invalid.
How is it different from zk rollups?
A zero-knowledge rollup, or zk rollup, posts a cryptographic proof with each batch. That proof shows the batch is valid without anyone having to challenge it. An optimistic rollup assumes the batch is valid and relies on fraud proofs instead. The difference affects withdrawal times and how hard the network is to build.
Frequently asked questions
Most optimistic rollups use ETH for gas, so you usually need ETH on that network. Check the network first, because a few use their own gas token.
Many Ethereum wallets let you add another network. Look for an add-network option in settings, then confirm the network name and chain ID on the rollup's official site.
Yes. A bridge bug or hack, or sending assets to the wrong network, can leave funds stuck or lost. Fraud proofs on the rollup do not protect the bridge.
No. Some are zk rollups that post validity proofs, and others are sidechains with their own consensus.
