Hydraulic systems offer a range of benefits, such as high power density, ruggedness, linear actuation, and low cost, which have led to the use of hydraulic actuation systems throughout a wide range of industries. However, they also typically suffer from lower efficiency than competing methods of actuation, which limits their appeal in some applications, and presents a threat to their continued prevalence as energy efficiency becomes increasingly important. The relatively low efficiency of hydraulic systems is due in part to inefficient components, such as pumps and motors, and partially due to the typical method of control, which uses throttling valves to place restrictions in the flow path to dissipate excess power as heat. Several approaches to reducing throttling losses have been studied, such as using pumps, motors, or transformers to control individual actuators, or creating circuits with multiple pressure levels to reduce the pressure drop across individual valves. However, these approaches often suffer from high cost and size requirements, or experience reduced control performance, which can limit their appeal. An alternative approach is to use high-speed on/off valves to charge and discharge energy storage elements to transform the power flow rather than restricting it. Different configurations of switching hydraulic circuits can be used to create variable pumps, motors, actuators, or transformers. This approach can significantly reduce the power loss that is typically associated with controlling hydraulic systems using conventional valves. However, control using high speed switching valves presents several challenges. The most significant drawback is the power ripple that results from the discontinuous nature of the control valves. While this can be mitigated using energy storage elements, such as inertias and accumulators, it cannot be completely eliminated. Furthermore, there is a fundamental trade-off between the degree of smoothing and the speed of response of the system. In chapter 2 the system dynamics of a switching system are studied in the context of a specific switching circuit, the Virtually Variable Displacement Pump (VVDP), and it is demonstrated that the trade-off between ripple and response time can be improved by increasing the Pulse-Width-Modulation (PWM) frequency and through the use of feedback control. An example of the VVDP circuit, which uses an on/off valve to load and unload a fixed displacement pump with an accumulator to smooth the output flow, is shown experimentally to improve the efficiency of the control system by up to 43% over a throttling valve system when operating at 2.5 Hz. However, this efficiency benefit is reduced to 24% when the switching frequency is increased to 10Hz. This highlights another challenge with designing switching circuits: the power loss mechanisms that occur in on/off valve controlled systems, particularly transition throttling and compressibility losses which occur every switch, can reduce the potential efficiency benefits and must be taken into account as PWM frequencies are increased. The throttling loss that occurs as the valve transitions between states is often the largest source of power loss in on/off valve systems operating at moderate to high PWM frequencies. The long transition time of many conventional switching valves places an upper limit on the PWM frequencies that can be achieved while still providing an efficiency benefit. To reduce the effect of the transition loss, the concept of hydraulic soft switching is introduced in chapter 3 which, in simulation, reduced the transition loss by 81%. This technique, which mimics a concept from switching electrical converters, provides temporary flow paths for the hydraulic fluid to bypass the transitioning on/off valve, thus avoiding most of the transition throttling. Another power loss that must be considered, particularly at high PWM frequencies, is the power needed to actuate the on/off valve. While increasing the PWM frequency can be effective at reducing the output ripple and the size of the energy storage elements, the energy needed to accelerate the switching elements in conventional on/off valves at high PWM frequencies is not insignificant. If the kinetic energy of the switching element is not re-captured, the power needed to actuate the valve increases with the PWM frequency cubed. To address this problem, a novel type of rotary valve that spins at a constant speed is presented in chapter 4. This rotary valve achieves high PWM frequencies by alternately connecting a fixed port in the housing to supply or tank as the spool rotates. The duty ratio, which describes the fraction of the rotation that the valve is “on,” is adjusted by moving the valve spool axially. The fact that the valve rotates with a constant speed eliminates the need to accelerate the valve element, meaning the valve must only overcome viscous friction. This has the potential to enable much higher switching speeds than conventional valves. However, with this novel valve architecture, there are a number of design trade-offs that must be negotiated. In chapter 4, the design of the rotary valve, in the context of both a VVDP and a Virtually Variable Displacement Pump/Motor (VVDPM), is formulated as a constrained power loss minimization problem to determine the optimal design parameters. For the VVDPM, this optimization is done for a device that is operating as the wheel motor in a Hydraulic Hybrid Passenger Vehicle. In addition to losses due to throttling across control valves, hydraulic system efficiency is degraded by power losses within hydraulic components, such as pumps and motors. The majority of these losses stem from leakage and friction caused by high pressure applied to elements that move relative to each other. In conventional piston type devices, which are common in the hydraulics industry, the displacement of the unit, and thus its power output, is varied by changing the stroke length of the pistons as they are rotated. This method maintains high pressure on the same number of pistons, regardless of the power output, resulting in leakage and friction losses that do not decrease with the power output and a sharp drop in efficiency at low displacements. In order to address this problem, a different form of on/off control can be used: direct control of individual pistons in a pump/motor. Using this method, pressure is removed from pistons to reduce the pump/motor displacement, which allows the power losses in the system to decrease with displacement. In chapter 5, the power loss mechanisms in both a conventional and discrete piston device are compared, and the potential efficiency benefit is highlighted. One challenge with creating discrete piston controlled pump/motors is designing the control valves to operate quickly and efficiently enough to provide effective control of the pressure in the piston chambers. Designs have been proposed that use two fast-acting electrohydraulic valves to connect the piston to supply or tank. This approach provides flexibility in selecting the disabling strategy and valve timing, but it also leads to designs that are expensive and complicated to control. The requirements for a discrete piston control valve are that it must switch quickly, especially as the pump/motor shaft speed increases, require little actuation power, and have repeatable timing with respect to the pump/motor shaft position. These characteristics match well with the two degree of freedom rotary valve described in chapter 4. In chapter 6 several valve designs based on the rotary valve concept are proposed, and a mechanism using the rotary valve as a pilot stage driving three-way spool valves is selected, analyzed, and designed as a part of a discrete piston controlled pump/motor. The detailed design of a discrete piston prototype, which uses hydro-mechanical rather than electrohydraulic control valves is described, and experimental results are presented in chapter 7. While high internal leakage caused by manufacturing challenges limited the experimentally demonstrated efficiency, estimates of the power loss without the internal leakage, analytical equations, and simulation results demonstrate the potential of the discrete piston control approach. Whether they are used to create switching circuits that avoid the losses associated with conventional throttling valves, or if they are used to reduce the leakage and friction losses in a discrete piston controlled pump motor, on/off valves can be used to improve the efficiency of hydraulic control systems. However, the design of switching valve controlled systems is not without challenges, and this thesis examines many of the potential benefits, as well as difficulties in creating efficient and effective solutions using on/off valves.