This paper presents a systems thinking approach to cost growth estimation for the Department of Defense weapon system life cycle costing. The research herein clarifies the effects of human behaviors, time delays, inaccurate expectations of cost growth behavior and other factors, offering improved conceptualization of the impact of abstract variables on cost growth. Methods of research include analysis of numerical data, statistical findings by research and development groups (RAND), Department of Defense system acquisition reports, and application of the developed model to a relevant program. A systems thinking approach to cost growth behavior analysis provides decision-makers with a holistic understanding of cause, effect and reaction in order to more realistically estimate cost growth. This paper provides a foundation for such an approach.