Optimistic Rollup
A Layer 2 scaling solution that assumes transactions are valid by default and uses fraud proofs to challenge invalid state transitions within a challenge window.
Optimistic Rollups execute transactions off-chain, post batched transaction data to Ethereum, and assume all transactions are valid unless a 'fraud proof' is submitted within a challenge window (typically 7 days). During the challenge window, any full node can submit a fraud proof demonstrating that a state transition was invalid. If a fraud proof succeeds, the invalid batch is rolled back and the sequencer is penalised.
The 7-day challenge window is the source of the primary user-visible limitation: withdrawals from Optimistic Rollups back to Ethereum take 7 days through the native bridge. In practice, liquidity bridges (Hop Protocol, Across) provide fast liquidity against this position, allowing users to exit in minutes — for a small fee — while the bridge provider waits out the challenge period.
Optimistic Rollups (Arbitrum One, OP Mainnet, Base) are EVM-equivalent, meaning they run the same code as Ethereum with minimal modifications. This made them the dominant L2 architecture in 2022–2023 when ZK-Rollup zkEVM implementations were still maturing. They remain the highest TVL category of L2s as of 2025.