This thesis presents an end-to-end approach for building computer networks that can be reasoned about and verified formally. In it, we present a high-level specification language for describing the desired forwarding behavior of networks based on regular expressions over network paths, as well as a tool that automatically verifies network forwarding policies; an approach to building formally verified compilers and runtimes for forwarding policies written in a network programming language that preserve the semantics of the source policy; and a technique for updating network configurations while preserving correctness.