Liquid ring vacuum pumps and compressors are mechanical positive displacement turbomachines that can operate in the mode of both vacuum compressor and vacuum pump. They are widely used in the fields of filtration, cryogenic, reaction, gas injection, condensation, evaporation, and convective drying. Despite their low performance, they have many advantages such as low costs, simplicity, reliability, improve suction characteristics, and in many cases, can provide a more efficient option. Improving efficiency is the key to decrease energy consumption and costs. The article analyzed and evaluated the previous investigations regarding these machines with a focus on challenges of improving efficiency, advances in numerical simulation, and experiments to afford beneficial insight on future research and next steps. The study revealed that the thermal interaction and the cavitation phenomenon are among the most important problems that require further research.