An optimistic rollup batches Ethereum transactions off the main chain and posts compressed transaction data to Ethereum, assuming all transactions are valid unless someone submits a fraud proof within the challenge window. The “optimistic” part is the default assumption of honesty: verification only happens if challenged. Arbitrum, Optimism, and Base are all optimistic rollups. Together, they hold more total value locked than any other Layer 2 category.
How optimistic rollups work
A sequencer node collects transactions from users, executes them off-chain, and posts a compressed batch to Ethereum as calldata. The batch includes state differences (what changed after all transactions executed) but no proof of correctness. Verifiers watch the batches and can submit a fraud proof if they find an invalid state transition. If no fraud proof is submitted within 7 days, the batch is considered final.
The 7-day challenge window is why withdrawals from an optimistic rollup back to Ethereum take one week by default. Bridge protocols like Across and Hop use liquidity providers to advance the funds immediately, charging a small fee (typically 0.05 to 0.10%) for taking on that waiting-period risk.
What this means for traders
Optimistic rollups support full EVM equivalence more easily than ZK-rollups, which is why Arbitrum and Base have the largest DeFi ecosystems among L2s. Arbitrum One holds over $15 billion in TVL as of 2026; Base grew rapidly as Coinbase’s native L2. For traders, the practical points are: gas fees 95% lower than mainnet, identical smart contracts (same protocols you know), and the 7-day official bridge delay which fast bridge aggregators work around at a small fee.
See: layer 2 crypto explained for the broader L2 context and ZK-rollups explained for comparison with the other main rollup type.
A concrete example
You bridge $10,000 USDC from Ethereum to Arbitrum via the official bridge. Within 10 minutes, your USDC is on Arbitrum. You use it in GMX to trade BTC perpetuals. Later you want to move it back to Ethereum. Official Arbitrum bridge: 7 days. Across Protocol: USDC arrives on Ethereum within 5 minutes, with a $6 bridging fee (0.06%). For routine transfers under $20,000, Across is the practical choice. For amounts in the hundreds of thousands, the official bridge’s lower tail risk may be worth the week.
Frequently asked questions
How does fraud detection work?
Any network participant can run a verifier node that re-executes batches locally and compares results against what was posted to Ethereum. If they find a discrepancy, they submit a fraud proof to the on-chain dispute contract. The sequencer’s staked collateral is slashed if fraud is confirmed. This economic incentive makes honest sequencing rational and fraud prohibitively expensive.
What is a sequencer and who runs it?
A sequencer is the node that orders and batches transactions. On Arbitrum, it is run by Offchain Labs; on Optimism and Base, by OP Labs and Coinbase respectively. A malicious sequencer could theoretically censor transactions but cannot steal funds (the 7-day window lets users exit). Decentralizing sequencers is an active development focus in 2026 for all major optimistic rollups.
Is Base different from Optimism?
Base is built on the OP Stack, Optimism’s open-source rollup framework, and operated by Coinbase. Transactions settle to Ethereum via the same optimistic fraud proof mechanism. The difference is Coinbase’s involvement: Base has deep integration with Coinbase’s existing user base and fiat on-ramp, which drove its rapid TVL growth in 2024 and 2025.
