Project Management and Systems Thinking overlap. In a software development world where the problems with today's legacy system are a consequence of yesterday's enterprise-wide solution, it is useful to understand cycles of cause and effect and their attendant time delays. As system complexity increases, systems thinking becomes increasingly important in generating debate and insight about an uncertain and changing environment. Surprisingly, project managers do not seem to use simple systems thinking tools even though these provide unique benefits in framing and solving problems that arise from multiple perspectives and relationships. The purpose of this paper is to introduce systems thinking and describe its application to the management of complexity in projects. It is based on a subject developed and taught by two of the authors if the paper in a Master of Project Management Course offered at the University of Technology Sydney to help project managers use systems thinking approaches to deal with project complexity. The subject evolved over a three year period using an action research approach to improve the subject to meet the needs of the students as well as the increasingly complex environment that they face in their projects. In this paper systems thinking is positioned in the evolution of the project management discipline, and its relationship to complex systems explored. Selected concepts are then described and applied to the three phases (concept, implementation and evaluation) of the system development lifecycle.