This book presents a comprehensive review of various aspects of the novel and rapidly developing field of active matter, which encompasses a wide variety of self-organized self-driven energy-consuming media or agents. Most naturally occurring examples are of biological origin, spanning all scales from intracellular structures to swimming and crawling cells and microorganisms, to living tissues, bacterial colonies and flocks of birds. But the field also encompasses artificial systems, from colloids to soft robots. Intrinsically out of equilibrium and free of constraints of time-reversal symmetry, such systems display a range of surprising and unusual behaviors. In this book, the author emphasizes connections between fluid-mechanical, material, biological and technological aspects of active matter. He employs a minimum of mathematical tools, ensuring that the presentation is accessible to a wider scientific community. Richly illustrated, it gives the reader a clear picture of this fascinating field, its diverse phenomena and its open questions.