Hydrocyclones are widely employed in both heavy and light industries. However, exact mechanisms underlying enhanced-separation technologies developed by optimizing operating parameters and conditions in hydro cyclones remain unclear. Accordingly, many research groups have conducted numerous investigations to expand the application range of hydrocyclones by optimizing operating parameters and conditions. This paper presents a comprehensive state-of-the-art review of the aforementioned hydrocyclone enhanced-separation technologies, which are classified into two groups: (i) operating parameters, including feed flow rate, feed pressure, feed density difference, feed particle parameters (concentration, size, shape, and arrangement), and feed fluid parameters (viscosity and rheology); and (ii) operating conditions, including electrical hydrocyclones, magnetic hydrocyclones (electromagnetic field and permanent magnetic field), magnetic fluids hydrocyclones, electro-chemical hydrocyclones, flocculant-assisted hydrocyclones, hydrocyclones enhanced by flotation, hydrocyclones enhanced by control particles; hydrocyclones enhanced by adjusting back pressure, and hydrocyclones enhanced by monitoring and automatic control. These enhanced-separation technologies were analyzed and summarized based on the critical separation-performance parameters, such as separation efficiency, cut size, split ratio, energy consumption, capacity, and separation sharpness. It is hoped that both reviewed contents and proposed challenges may be helpful to the researchers and eventually yield some perspective knowledge, which results in the improvement of economic feasibility of separation by hydrocyclones.