The number of blockchains in existence continues to rise. The value is still concentrated on a few chains, but the diversity has increased in recent years (Figure 1). Put differently, the future is modular and multi-chain. In such world, blockchains need to be able to talk to each other to avoid fragmentation. Blockchain bridges enable this. Initially focused on moving tokens across blockchains, solutions have emerged that allow for more generic cross-chain communication. There are now dozens...