The newly emerged software defined system (SDS) promises a new information system resource allocation and management way. The main concept of SDS is to separate the control plane from the data plane in various subsystems, e.g., sensing, communication, networking, and computation. By such means, various resources of the information system are virtualized and therefore can be managed in a more friendly and flexible manner. It is widely believed that SDS is able to lower the barrier for system and application innovation, and will become an inevitable trend towards the future generation of the information system. In this chapter, we first identify the emergence and then give an overview as well as the main concepts of SDS. Regarding that many enabling technologies are already available to realize the vision of SDS, we also present a brief summarization of the main enabling technologies for SDS and identify their key characteristics.