A bridge is the mechanism that lets a token created on one blockchain (say, USDC on Ethereum) appear and be usable on another (USDC on Base, Polygon, Solana). The most common pattern: lock the original token in a contract on the source chain, then mint a wrapped version on the destination. Burning the wrapped version unlocks the original. Bridges have been the most-attacked component of crypto, with multi-hundred-million-dollar exploits across 2021-2024.
Trusted bridges rely on a centralized custodian or federation. Trust-minimized bridges use light clients (the destination chain verifies proofs of source-chain state) or canonical bridges built into rollup protocols. Optimistic bridges use challenge windows; ZK bridges use validity proofs.
Bridge fees, latency, and security models vary widely. Native rollup bridges (the ones built into Arbitrum, Optimism, Base) are the most secure because they inherit Ethereum's security. Generic cross-chain bridges (LayerZero, Wormhole, Axelar) are more flexible but historically more attacked.
Multi-chain crypto cannot exist without bridges, and bridges are the weakest security link. Choosing which bridge to use, and how much value to commit to it, is one of the more consequential decisions in DeFi.
L2 bridge flows are visible in the exchange-flows premium endpoint, which tracks labeled wallets across L1 and L2 chains.