Software-defined networks (SDNs) are a new kind of network architecture in which a controller machine manages a distributed collection of switches by instructing them to install or uninstall packet-forwarding rules and report traffic statistics. The recently formed Open Networking Consortium, whose members include Google, Facebook, Microsoft, Verizon, and others, hopes to use this architecture to transform the way that enterprise and data center networks are implemented. In this paper, we define a high-level, declarative language, called NetCore, for expressing packet-forwarding policies on SDNs. Net-Core is expressive, compositional, and has a formal semantics. To ensure that a majority of packets are processed efficiently on switches-instead of on the controller-we present new compilation algorithms for NetCore and couple them with a new run-time system that issues rule installation commands and traffic-statistics queries to switches. Together, the compiler and run-time system generate efficient rules whenever possible and outperform the simple, manual techniques commonly used to program SDNs today. In addition, the algorithms we develop are generic, assuming only that the packet-matching capabilities available on switches satisfy some basic algebraic laws. Overall, this paper delivers a new design for a high-level network programming language; an improved set of compiler algorithms; a new run-time system for SDN architectures; the first formal semantics and proofs of correctness in this domain; and an implementation and evaluation that demonstrates the performance benefits over traditional manual techniques.