What is a complex system? Although “complexity science” is used to understand phenomena as diverse as the behavior of honeybees, the economic markets, the human brain, and the climate, there is no agreement about its foundations. In this introduction for students, academics, and general readers, the authors develop an account of complexity that brings the different concepts and mathematical measures applied to complex systems into a single framework. The book begins with an overview and a brief history of complexity science. Complexity science is relatively new but already indispensable. Many of the most important problems in engineering, medicine, and public policy are now addressed with the ideas and methods of complexity science. The conceptual foundations of complexity science are disputed, and there are many and diverging views among scientists about what complexity and complex systems are. Its origins lie in cybernetics and systems theory and it is related to dynamical systems theory and the study of cellular automata. The book introduces the different features of complex systems and discusses different conceptions of complexity with the authors documenting their own account. In do so, they explain why complexity science is so important in today's world.