A scaling solution built on top of a Layer 1 blockchain to increase speed and reduce costs while inheriting its security.
A Layer 2 (L2) is a separate blockchain or protocol that operates on top of a Layer 1 (like Ethereum) to increase transaction throughput and reduce fees while inheriting the L1's security guarantees. L2s batch hundreds or thousands of transactions together and submit compressed proofs back to the L1. The main L2 types are Optimistic Rollups (Arbitrum, Optimism, Base — assume transactions are valid unless challenged) and ZK-Rollups (zkSync, Starknet, Scroll — use zero-knowledge proofs to cryptographically verify transactions). Ethereum's scaling roadmap centers on L2s, and they now process more transactions than Ethereum mainnet itself. L2s have their own ecosystems of DeFi protocols, NFT marketplaces, and applications, while ultimately settling on Ethereum for final security. The tradeoff is added complexity for users (bridging assets) and varying degrees of decentralization (sequencer centralization is an active concern).
Layer 2 solutions are scaling technologies built on top of Layer 1 blockchains that process transactions off-chain while inheriting the security of the underlying L1. On Ethereum, L2s like Arbitrum, Optimism, Base, and zkSync have dramatically reduced transaction costs (from $5-100+ to $0.01-0.10) while maintaining Ethereum's security guarantees. L2s work by bundling thousands of transactions, executing them off-chain, and posting compressed proofs or data back to the L1. There are two main L2 architectures: Optimistic Rollups (Arbitrum, Optimism) assume transactions are valid unless challenged within a dispute window, while ZK-Rollups (zkSync, Starknet) use zero-knowledge proofs to mathematically verify transaction validity. Ethereum's roadmap explicitly delegates execution to L2s — the L1 focuses on security and data availability while L2s handle user-facing transaction processing. This 'rollup-centric' vision has attracted billions in TVL to L2 ecosystems.
Arbitrum processes over 2 million transactions per day — more than Ethereum mainnet — while Ethereum transaction fees are typically 10-50x cheaper on Arbitrum, inheriting Ethereum's security through optimistic fraud proofs.
For most DeFi activity, yes. L2s offer 90-99% lower fees with comparable security to mainnet. Major protocols (Aave, Uniswap, Curve) are deployed on L2s. Use mainnet for activities that require the deepest liquidity or for interacting with protocols not yet on L2s. Bridge assets to L2s using official bridges for the safest experience.