Overview
An optimistic rollup is a Layer 2 scaling solution that processes transactions off-chain and assumes they are valid by default, relying on fraud proofs to catch errors. Because the system is "optimistic" that published batches are honest, anyone can challenge a suspicious batch during a dispute window. Arbitrum and Optimism are the best-known optimistic rollups.
How It Works
A sequencer executes transactions and posts the resulting state root and compressed data to Ethereum. Validators monitor the chain and can submit a fraud proof if they believe a batch is incorrect; an interactive game then isolates the exact dispute, and an on-chain judge rules on it. If fraud is proven, the offending operator is penalized and the invalid state is reverted. Withdrawals therefore wait for the challenge window to expire.
Why It Matters
Optimistic rollups offer full EVM compatibility and lower operating costs than ZK-rollups, making them a pragmatic home for existing Ethereum applications. Their trade-off is withdrawal delay and the assumption that at least one honest actor will challenge bad behavior. As of 2026 they host the majority of DeFi activity on Ethereum L2s.
Related Concepts
Optimistic rollups are one branch of the Rollup family, contrasted with ZK-Rollups. Their security rests on Fraud Proofs, and they are typically built on the OP Stack, connecting to Layer 1 via Bridges.