Active colloids are self-propelled particles, powered by energy harvested from the environment. This field of research has been growing over the past 20 years, attracting researchers from multiple disciplines. Biomedical engineers seek to harness the abilities of motile bacteria, materials chemists are fascinated by the concept of synthetic particles becoming autonomous and the new opportunities this presents, and soft matter physicists see active colloids as a model system for active matter, unravelling the principles of nonequilibrium systems. Beginning with the fundamentals, this book discusses the various types of active colloids, classified by energy source, as well as microbial active colloids. Several chapters are dedicated to theory and modelling, followed by an exploration of major developments and research frontiers. With expert contributions from around the world, this book is a useful reference and a source of inspiration for new and experienced researchers.