Hydrostatic Pressure Machines (HPM), are a class of hydropower energy converter designed to operate at sites with heads below 3 meters; sites receiving increasing interest as the demand for power from renewable energy sources grows. The HPM is a ‘Pressure machine’, applying the pressure produced by differing water levels at a site, directly to the blades of the device to extract power. Prior to the current research, these machines had only existing as laboratory models. This thesis describes the design, construction and testing of a 5 kW prototype HPM installed at a re-activated mill site in Bavaria. Observations and performance test results from this full scale unit are then compared with the results of scale model tests carried out in the laboratory. New theory is developed to account for the geometry of the prototype machine and the variations in water levels encountered during operation. This is found to give very good agreement with performance measurements from both prototype and model tests, with no scale effects identified between the scales over the normal operating range of the machine. Several alternative rotor designs are tested at model scale, which demonstrate useful performance gains compared with the prototype machine. Direct blade force and cell pressure measurements are also obtained during model operation which has increased our understanding of the energy exchanges taking place between rotor and fluid within the machine. This in turn helps to identify the key machine geometries which impact performance.