In this dissertation, both the ideal and non-ideal torque on the cylinder block of an axial-piston swash-plate type hydrostatic pump are examined. Traditionally, the torque on the cylinder block has been estimated by using a standard theoretical-expression that does not consider the compressibility of the fluid or the frictional characteristics within the pump. Furthermore, past research that has considered fluid compressibility and friction has been conducted on a macroscopic level and simply emphasizes test results without offering sufficient explanations for them. As the need for improved efficiency of hydrostatic pumps increases, an improved understanding of the torque characteristics within these machines is required;This research compares the results of a detailed mathematical-model with actual test-data for the torque on the cylinder block of an axial-piston swash-plate type hydrostatic pump. As a result of this analysis, an improved model for the idealized torque on the cylinder block is offered in closed-form. Furthermore, a numerical program is written to calculate the net torque on the cylinder block which includes the torque that is lost due to friction. Using the numerical model, the torque losses are separated for each component within the machine and compared with each other to determine where the torque losses are greatest. Lastly, basic design parameters are numerically varied by small amounts to determine the magnitude of influence that specific design changes might have and it is shown that some design changes have greater influence on the torque loss than others.