International audience ; Efficiency improvement is a key issue in any machine. The fluid power industry relies on volumetric pumps which need to pump high pressure fluid to a set of actuators located at different positions in a given device; the higher the pressure, the smaller the actuators need to be, therefore allowing to reduce the weight of the machine, being this, a critical issue in any flying device. At the present, the pumps which are able to produce the highest fluid pressure are piston pumps, and among the different sort of piston pumps, axial piston pumps seem to be the most widely used, probably due to its high efficiency and reliability. Pumps and motors overall efficiency, is in reality the product of volumetric, mechanical and hydraulic efficiency, therefore a decrease in any of these efficiencies will bring an overall efficiency decrease. In this book chapter, a deep study on the different axial piston pump moving parts shall be presented, equations clarifying leakage and pressure distribution in all axial piston pump moving parts will be introduced, and the dimensional parameters from which leakage depends will be clearly defined. As a result, a tool to improve piston pump volumetric efficiency shall be established. To validate the equations presented, a comparison between results produced by the equations, by several CFD models of each axial piston pump moving parts and several experimental measurements will be performed. Thanks to this comparison, the validity limits of the equations presented will be established. Thanks to the theory developed and the different test rigs used, a better understanding of the slippers dynamic behavior and barrel dynamics was gathered, pressure distribution, forces and torques generated in the slipper-swash plate, barrel-port plate and piston barrel will be presented, comparisons between CFD, analytical equations and experimental results will validate the new theory produced. One of the newest characteristics of the analytical, CFD and experimental development ...