A layer 2 (L2) is a network built on top of a base blockchain (the layer 1, such as Ethereum) to make transactions faster and cheaper, while still relying on the base layer for security. Transactions are executed on the layer 2, and the results, together with the data or proof needed to verify them, are posted back to layer 1. Users get low fees and quick confirmations, and their funds remain protected by Ethereum. Well-known Ethereum layer 2 networks are Arbitrum, Optimism, Base, zkSync, Starknet, Scroll and Linea.
Why layer 2 exists
Ethereum's mainnet processes a limited number of transactions per block. When demand is high, gas fees rise sharply, which makes small transactions and many applications impractical. Instead of making the base layer itself much bigger, which would make running a node harder and hurt decentralisation, Ethereum's roadmap moves most activity to layer 2 networks. Since the Dencun upgrade in 2024 introduced cheaper "blob" data for layer 2s, their fees have dropped to a few cents or less.
Types of layer 2
- Optimistic rollups: assume transactions are valid unless someone proves otherwise with a fraud proof during a challenge period. Arbitrum, Optimism and Base work this way. Withdrawals to Ethereum via the official bridge take about a week.
- ZK rollups: post a cryptographic validity proof with every batch, so Ethereum can verify correctness directly. zkSync, Starknet, Scroll and Linea work this way, with faster final withdrawals.
- Validiums and other variants: keep transaction data off Ethereum for even lower costs, at the price of weaker guarantees.
- Sidechains: separate blockchains with their own security that connect via a bridge. Strictly speaking they are not layer 2, because they don't inherit Ethereum's security.
See what is a rollup for how rollups work in more detail.
What to consider when building on layer 2
Most layer 2s are EVM-compatible, so smart contracts and tools for Ethereum work with few or no changes. Points to check are the maturity of the network (many still rely on a single sequencer and on upgrade keys controlled by the team), withdrawal times, available liquidity and users, and support from wallets, oracles and indexers. See our layer 2 development, for example on Arbitrum, Base and zkSync.
Frequently Asked Questions about Layer 2
What is the difference between layer 1 and layer 2?
Layer 1 is the base blockchain that provides security and final settlement, such as Ethereum or Bitcoin. Layer 2 is built on top of it to process transactions more cheaply and quickly, and settles back to layer 1.
Are layer 2 networks safe?
Rollups inherit much of Ethereum's security, but many are still maturing: a single sequencer can delay transactions, and upgrade keys can change the system. Independent trackers rate how decentralised each layer 2 is.
Is Polygon a layer 2?
Polygon PoS has historically been a sidechain with its own validators. Polygon also develops ZK technology for layer 2 scaling. Whether a network counts as a layer 2 depends on whether it inherits Ethereum's security.
Does Bitcoin have layer 2s?
Yes. The Lightning Network is the best-known Bitcoin layer 2, aimed at fast, cheap payments through payment channels.