The digital displacement hydraulic piston pump is a hybrid device which combines a microcompressor with an established form of high-pressure pump to create a highly integrated machine which can produce a variable high-power output directly from an electronic command. The actively controlled inlet poppet-valve in each cylinder can be held open against the discharging flow in order to disable it during a single cycle. Cylinders can be disabled in this manner, following a maximally smooth sequence, allowing a controlled output flow to be achieved. A compliant device located near the pump, such as an accumulator, provides time-averaging of the flow pulsations in order to minimise the effects of the quantisation error caused by cylinder disabling. The advantages of this approach over the conventional variable-swash axial piston pump lie with both the response speed and the inherent energy efficiency of real-time cylinder selection. Disabling cylinders in this way restricts parasitic losses to very low levels since unused cylinders are not pressurised nor do they incur loads on their associated bearings. The response time of the pump is related to shaft speed, with the pump able to attain either full or zero output from any starting condition, in less than a single shaft revolution. At induction motor speeds this allows large-signal response times of the same order as those achieved by commercial proportional valves. The thesis chronicles the development of the Digital Displacement pump. It begins with the formulation of a simulation model which is able to predict the behaviour of the machine in both flow and pressure control modes. The valve control possibilities are then explored and the design of active valve latches using finite-element analysis described. The sinusoidal flow forces on the disabled poppet are evaluated through a large range of experiments and the results condensed into parametric equations useful for predicting the valve latching requirements of most machines. The mechanical and electronic hardware design, leading to the construction of the prototype, is then discussed.