Configuration changes are a common source of instability in networks, leading to broken connectivity, forwarding loops, and access control violations. Even when the initial and final states of the network are correct, the update process often steps through intermediate states with incorrect behaviors. These problems have been recognized in the context of specific protocols, leading to a number of point solutions. However, a piecemeal attack on this fundamental problem, while pragmatic in the short term, is unlikely to lead to significant long-term progress. Software-Defined Networking (SDN) provides an exciting opportunity to do better. Because SDN is a clean-slate platform, we can build general, reusable abstractions for network updates that come with strong semantic guarantees. We believe SDN desperately needs such abstractions to make programs simpler to design, more reliable, and easier to validate using automated tools. Moreover, we believe these abstractions should be provided by a runtime system, shielding the programmer from these concerns. We propose two simple, canonical, and effective update abstractions, and present implementation mechanisms. We also show how to integrate them with a network programming language, and discuss potential applications to program verification.