The second edition of this introductory book presents the principles of hydrostatic transmissions. The basic concepts of typical open-circuit and closed-circuit HSTs are described in a simple-to-understand manner. The configurations, types, specifications, and applications of HSTs are also given. Appendix 2 contains case studies on an open-circuit hydraulic concrete pump system, a hydraulic steering system, and typical displacement controls of a bidirectional variable-displacement axial-piston pump. Appendix 4 describes basic displacement control methods of variable-displacement axial-piston pumps. The book uses the SI system of units. Hydrostatic transmissions (HSTs) occupy a distinct place in fluid power technology. They are widely used in agricultural tractors, on and off-highway mobile equipment, and various self-propelled equipment for the transfer and control of power. An HST provides an infinitely variable speed between zero and maximum in both forward and reverse modes of operation without changing the prime mover's speed. Moreover, manufacturers are bringing out smaller, lighter HSTs with advanced electronic controls and improved performance. These factors make HSTs a cost-effective choice for many industrial and mobile applications.