Abstract: A multi-stage hydraulic rotating machine (MSHRM) maintains near-optimal efficiency over widely varying conditions of service (COS) when controlling a fluid having a gas volume fraction (GVF) greater than 50% and large changes in volumetric flow rate (VFR) between stages. The MSHRM includes separately controlled stages having at least two different designs with different VFR ranges. Stage impellor differences can include impellor diameter, blade pitch, blade width, blade number, inlet diameter, and outlet diameter. Diffusers can differ in similar ways between stages. VFR ranges can be progressively higher or lower in successive stages. The stages can share a common VFR range within which incompressible liquids can be controlled. The MSHRM can function as a pump or turbine, and can be applicable to energy storage and recovery in “green” energy systems.