This report describes the adoption of Cleanroom Software Engineering Methodology into a mature software organization as defined by the SEI's Capability Maturity Model. The organization, a defense contractor operating at SEI Level 4/5, has been working to develop a large real time system under tight budget and schedule constraints. This overview describes how Cleanroom principles were incorporated into an existing, mature software development framework and outlines some of the techniques used to facilitate the transition from a conventional to a Cleanroom development effort. In addition, we also describe issues related to impact and technology insertion.