A scaling solution built on top of a base blockchain (Layer 1) that processes transactions off-chain and settles them in batches, reducing fees and increasing throughput.
Ethereum's main chain (Layer 1) processes roughly 12–15 transactions per second and has limited block space, causing fees to spike during congestion. Layer 2 solutions address this by moving transaction execution off the main chain, then periodically 'rolling up' proofs or transaction data back to Ethereum for settlement. This inherits Ethereum's security while dramatically increasing capacity and reducing per-transaction costs.
The two main rollup architectures are Optimistic Rollups (Arbitrum, Optimism, Base) - which assume transactions are valid by default and allow a challenge period for fraud proofs - and ZK-Rollups (zkSync, StarkNet, Polygon zkEVM) - which generate cryptographic validity proofs for every batch, providing instant finality. ZK-Rollups have stronger theoretical security but are technically more complex; Optimistic Rollups have a 7-day withdrawal delay back to Ethereum as a result of the challenge window.
For active DeFi traders, Layer 2 networks offer Ethereum-compatible DeFi applications at USD 0.01–0.10 per transaction versus USD 5–50 on mainnet. Major DeFi protocols (Uniswap, Aave, Curve) are deployed on multiple L2 networks. The trade-off is that bridging assets from Ethereum to an L2 - or between L2 networks - incurs time delays and bridge smart contract risk.
Worked Example
A liquidity provider manages an ETH/USDC position on Uniswap. On Ethereum mainnet, each rebalance transaction costs ~USD 40 in gas; they rebalance 8 times per month, spending USD 320. Running the same strategy on Arbitrum (a Layer 2 rollup), the same transactions cost ~USD 0.15 each - USD 1.20 total per month. The USD 318 monthly gas saving directly improves the strategy's profitability, changing it from borderline to comfortably positive after accounting for pool fees and impermanent loss.