Nomad is a new design for radically cheaper cross-chain communication without header verification. We expect operating Nomad to cut 90% of gas costs compared to a traditional header relay. To accomplish this, we took inspiration from optimistic systems (a la Optimistic Roll-ups). Nomad features many of the features we prize in an optimistic mechanism, like public verification, low gas fees, broad participation, but has a slightly different security model. Nomad will form the base layer of a cross-chain communication network that provides fast, cheap communication for all smart contract chains and rollups. It relies only on widely-available cryptographic primitives (unlike header relays), has a latency of thirty minutes (rather than an ORU’s one week latency), and imposes only about 120,000 gas overhead on message senders.