The Army Secure Operating System (ASOS) is a family of operating systems intended to serve the tactical needs of the US Army. It currently comprises two systems designed to be certifiable to classes C2 and A1 of the DoD Trusted Computer System Evaluation Criteria (TCSEC). Both operating systems provide a common user interface, support real-time applications written in Ada, and are configurable and easily portable to alternate hardware architectures. A brief overview of the development is presented, including a description of the hardware/software architecture, the current status of the project, and a review of lessons learned during the three year development.>