This introductory chapter provides an overview of power transmission in a hydraulic system. Unlike mechanical and electrical power transmission, hydraulic power transmission uses pressurized fluids to deliver energy and employs hydraulic cylinders or motors to convert the delivered energy into a mechanical form to perform useful work. The unique feature of hydraulic power transmission is that its pressurized fluids can be formed into any geometric shape, depending on the container, and delivers the power in all directions as needed. This makes it possible to place a hydraulic power system in a contained space and to make it act like the muscle in a human body to move loads in required patterns. This chapter also briefly introduces the theoretical fundamentals of hydraulic power transmission.