In the past, the administration of fluids in the perioperative period and in the critical care setting has been largely based on anecdote and/or flawed studies rather than sound evidence. Understanding of body fluid kinetics is rapidly advancing, and recommendations for fluid therapy are changing to keep pace with these developments. This chapters looks at fluid distribution and composition within the body, physical principles and measurements, recognition of fluid deficits, types of fluid loss, routes of fluid administration, types of fluid, blood transfusion, equipment for fluid therapy and blood tranfusion, fluid therapy for anaesthesia, monitoring fluid therapy.