A rollup is a scaling solution that processes transactions on a separate chain but posts cryptographic proof (or optimistic-challenge data) back to a base chain — almost always Ethereum — that validates the rolled-up state.
Two main flavors: - **Optimistic rollups** (Arbitrum, Optimism, Base): assume transactions are valid; a 7-day fraud-proof window lets anyone challenge invalid batches. Cheap to operate, slower finality to the base chain. - **ZK rollups** (zkSync Era, Starknet, Polygon zkEVM, Scroll): attach a cryptographic proof that the state transition is valid. Faster finality, higher proving cost.
Why rollups matter: they give Ethereum-equivalent security at a fraction of the gas cost. Typical rollup transactions in 2026 cost sub-cent to a few cents vs $1-20 on Ethereum L1. They scale Ethereum without changing Ethereum itself.
Market leaders in 2026: - Base (Coinbase, OP Stack): dominant consumer activity - Arbitrum: deepest DeFi liquidity - Optimism: OP Stack ecosystem hub - Polygon zkEVM, Scroll, Linea: ZK contenders
All of them inherit Ethereum's security via fraud or validity proofs. The sequencer (who orders transactions) is currently centralized on all major rollups in 2026 but decentralization is on every roadmap.