This document provides an overview of systems thinking in public health. It discusses types of systems including simple, complicated, chaotic, and complex adaptive systems. It describes WHO building blocks for health systems strengthening and limitations of this approach. Feedback loops, path dependence, scale-free networks, emergent behavior, and phase transitions are discussed as concepts from complexity science. Theories of systems thinking like catastrophe theory, chaos theory, and systems dynamics modeling are outlined. The document concludes with an example of a safe deliveries pilot project in Uganda that demonstrated emergent behavior and a phase transition.