This document discusses complexity science and complex adaptive systems. It defines complexity and distinguishes between simple, complicated, and complex systems. Complex systems have many interrelated and autonomous parts that interact in non-linear ways, making their behavior hard to predict. The document introduces the Cynefin framework for categorizing systems and describes properties of complex adaptive systems, including emergence and self-organization. It emphasizes that the whole of a complex system is greater than the sum of its parts and advocates developing a complexity mindset to understand and leverage complexity.