Optimistic Rollup

An optimistic rollup is a layer-2 network that scales a base blockchain by posting batches of transactions to it while assuming — optimistically — that the batches are valid, and relying on a challenge period during which anyone can prove fraud. Instead of verifying every transaction on the expensive base layer, the rollup executes them off-chain and publishes the transaction data plus a claimed resulting state to Ethereum. If the claim is wrong, an honest observer submits a fraud proof, the bad batch is rejected, and the dishonest party's bond is slashed.

This "innocent until proven guilty" model is why withdrawals from optimistic rollups to Ethereum traditionally take about seven days: the network must wait out the challenge window before treating a batch as final. In practice, users usually sidestep the delay with third-party bridges that front them liquidity for a fee. Arbitrum, Optimism, and Base are the leading examples; a swap that costs dollars on Ethereum mainnet typically costs cents on any of them, with the underlying data still settling to Ethereum.

A common misconception is that the seven-day delay applies to using the rollup; transactions on the rollup itself confirm in seconds — the wait applies only to trust-minimized withdrawals back to layer 1. The design's security depends on at least one honest, attentive verifier existing and on the data needed to check batches being available on-chain, and its main open criticisms are the long exit window and the centralized sequencers most implementations still run.