This book deals with the principle calculations of the hydraulic power systems and with special attention it focuses on the thermal-hydrodynamic aspect of the hydraulic system to build a good theoretical background, which should enable the students to analyse the static and dynamic performance of the different hydraulic power elements and systems. Also, this book deals with expert system technology as a tool, designed to assess hydraulic systems in thermal-hydrodynamic aspect before construction them by selecting a suitable component such as hydraulic reservoir and prime mover in term of dimensions, output power or allowed pressure.