Abstract: This invention, a variable displacement sliding vane pump/hydraulic motor, incorporates an eccentric chamber in which a rotor with a plurality of axially and radially moveable sliding vanes rotates. Axial positioning of the sliding vanes is accomplished by positioning a sliding wall; the swept volume and therefore the capacity of this invention is also directly related to the positioning of the sliding wall. This sliding wall can be positioned by external means including conventional manual and automatic devices. Various embodiments permit pumping capacity variability from essentially zero flow to full flow without the use of bypasses.