“Software-defined” has become one of the hottest buzzwords in both academia and industry. It describes a family of technologies including SDNs, SDS, and SDDCs that are collectively sometimes referred to as “softwaredefined everything” (SDX). In a softwaredefined system, hardware resources can be virtualized and managed by OS routines or the control plane, and users can write programs to access and manage the services provided by virtualized resources.4 We argue that OSs offer these same capabilities. For example, a traditional OS such as Linux or Windows provides virtualization of hardware resources through hardware drivers, and application development and runtime support through software development kits (SDKs) and libraries. Mobile OSs such as Android provide an extra software-defined layer with higher-level abstractions for mobile apps, including management of mobile data (for example, contact and location data), a set of APIs for app development, and a set of libraries to support app execution. Whether OSs run on small devices (such as TinyOS5 ) or huge clusters (such as cloud OSs), they offer both resource virtualization and function programmability. Put another way, a “software-defined” technology is really just an OS for that technology. In an SDN, for example, the control plane provides the ability to write high-level applications to manage the networking functions, while the data plane virtualizes networking resources. In sum, as Figure 1 shows, OSs and software-defined systems are mostly based on the same principles.