This chapter, Complexity and Simplicity, introduces the fundamental conceptual dynamics at play in climate security and in the development of a climate security intelligence capability. The chapter begins by defining what a system is and how a systems approach can benefit the understanding of climate security and a climate security intelligence. The discussion of systems naturally triggers the introduction of the concept of complexity, a phenomenon founded within the physical, biological, and chemical sciences. Complexity has been applied to the social sciences so this transition is discussed and why it is justified from a knowledge understanding perspective. The application of complexity is then discussed as it applies to policy formulation. Given the reality of complexity and the challenges therein within social science and policymaking, it is fundamentally necessary to raise perhaps the counterintuitive issue of simplification. The reason for simplification is that complexity requires specialization, and decision-makers are typically not specialists. The chapter concludes with examples of simplification efforts.