The term active fluids refers to motions that are created by transforming energy from the surroundings into directed motion. There are many examples, both natural and synthetic, including individual swimming bacteria or motile cells, drops and bubbles that move owing to surface stresses (so-called Marangoni motions), and chemical- or optical-driven colloids. Investigations into active fluids provide new insights into non-equilibrium systems, have the potential for novel applications, and open new directions in physics, chemistry, biology and engineering. This book provides an expert introduction to active fluids systems, covering simple to complex environments. It explains the interplay of chemical processes and hydrodynamics, including the roles of mechanical and rheological properties across active fluids, with reference to experiments, theory, and simulations. These concepts are discussed for a variety of scenarios, such as the trajectories of microswimmers, cell crawling and fluid stirring, and apply to collective behaviours of dense suspensions and active gels. Emerging avenues of research are highlighted, ranging from the role of active processes for biological functions to programmable active materials, showcasing the exciting potential of this rapidly-evolving research field.