Software is the foundational building material for the engineering of systems, enabling almost 100% of the integrated functionality of cyber physical systemsﾗespecially mission- and safety-critical software reliant systemsﾗto the extent that these systems cannot function without software. As a result, it is imperative that the DoD has the capability and capacity to affordably sustain software-reliant systems and to continually operate and achieve mission success in a dynamic threat, cybersecurity, and net-centric environment. The Carnegie Mellon University (CMU) Software Engineering Institute (SEI) has been performing studies to inform Departmental decisions regarding software sustainment policies and programs regarding complex weapon systems. These studies were based on interviews and discussions with sustainment centers across all of the Services, case studies on selected programs, and a literature review. In this paper we present an overview of our initial study regarding the DoDﾒs organic software sustainment infrastructure and its key components related to complex weapon systems, and a selection of key themes from our analysis of sustainment practices. There are two key takeaway messages. First, software sustainment is not effectively described with a model based on hardware (where sustainment can be treated as a discrete series of activities intended to restore form, fit, and function). Secondly, software sustainment is really about continuous engineering in which the software undergoes a series of engineering activities intended to deliver the latest capability to the warfighter, a task which is never ﾓdone.ﾔNaval Postgraduate School Acquisition Research Program