(1) Feed flow rate. With the increase in the feed flow rate, the se- paration efficiency and the split ratio increase slower and slower, whereas the pressure drop increases faster and faster. Then feed flow rate levels out over the operating range of hydrocyclones. Because further increase in feed flow rate will lead to an un- acceptable high pressure drop. What is worse, for de-oiling hydro- cyclones, further increase in feed flow rate will also ultimately in- duce the separation efficiency to decrease acutely.(2) Feed pressure. Feed pressure is proportional to the separation effi- ciency, energy consumption, and capacity. Nevertheless, the smaller the cylindrical diameter, the larger the effects of feed pressure on separation performance. As a result, (i) for hydro- cyclones with not too small cylindrical diameter, the increase in feed pressure cannot enhance the hydrocyclone separation sig- nificantly; while (ii) for mini-hydrocyclones, the feed pressure up to 6 MPa can help to realize solids classification in submicron range. In addition, to date, most high-pressure hydrocyclones have been fitted to heavy and fine feed materials with densities over 2000 kg/ m3 and diameters under 212 μm; whereas most low-pressure hy- drocyclones (1.38–5.56 kPa) have been developed and fitted to the materials with wide range of particle sizes and low densities, such as solid wastes in recirculating aquaculture system and foulant in sewage/wastewater.(3) Feed density difference. Feed density difference is proportional to the separation efficiency. Hence, the separation of low-density- difference material is one of the main challenges for hydrocyclones, which results in that the amount of the near gravity material fed into hydrocyclones is a critical factor causing problems, such as surging phenomenon, underflow orifice/vortex finder overloading, and flow-structure instability. Moreover, the effect of the feed particle density should be considered for selecting hydrocyclones size. The reason is that more ultrafine particles are misplaced to underflow in smaller hydrocyclones, whereas more coarse light particles are likely to unexpectedly flow to the overflow in larger hydrocyclones.(4) Feed particle parameters. With respect to the feed particle con- centration, for solid-liquid separation, feed concentration is in- versely proportional to the separation efficiency and separation sharpness, whereas is proportional to the cut size and pressure drop. While for the separation of immiscible liquids, the increase of feed concentration increases the separation efficiency owing to the in- creasing dominance of droplet coalescence at high feed con- centrations.