A bridge is infrastructure that transfers tokens or messages between two different blockchains. Since blockchains cannot natively talk to each other, bridges lock tokens on the source chain and mint a representation on the destination chain — or use a more complex proof-of-state mechanism.
Bridge types: - **Lock-and-mint**: tokens locked on chain A, wrapped version minted on chain B. Most common, most hacked. - **Burn-and-mint**: tokens burned on chain A, reminted on chain B. Circle's CCTP for USDC uses this. - **Liquidity-pool** bridges (Stargate, Across): pools on both sides; atomic swap via off-chain relayer. - **Optimistic** bridges (Nomad, Across): messages assumed valid with a challenge window. - **ZK bridges**: cryptographic proofs of source-chain state; most secure, highest complexity.
Bridges are the single biggest attack surface in DeFi history. Ronin ($625M, 2022), Wormhole ($320M, 2022), Nomad ($190M, 2022), and Poly Network ($611M, 2021) collectively lost more than $1.7 billion to bridge exploits.
For retail: use native bridges (Arbitrum's official bridge, Optimism's official bridge) for moves to/from Ethereum. Use CCTP for USDC across chains. Use smaller specialized bridges (Across, Hop, Relay) only for amounts you can afford to lose.