(1) The possibility and effects of the discrete-phase (e.g., liquid dro- plets, air bubbles, and solid particles) breakup and coalescence in various hydrocyclones;(2) The real reason for the decline in the separation efficiency at feed flow rates above Qmax, after which the separation efficiency drops dramatically;(3) Effects of various operating parameters and conditions on the discharge capacity of the apex and the vortex finder;(4) Whether “fish-hook effect” is a scientifically significant physical phenomenon or just random and sporadic phenomenon;(5) More effective shape factors or shape-factor combinations should be developed, optimized, and selected to investigate the effects of the arbitrary shape of feed particles as accurately and compre- hensively as possible;(6) More investigations on effects of the feed-continuous-medium rheology and the feed-dispersion-medium rheology on various separation-performance parameters;(7) Development and application of more compound-force-field hy- drocyclones, such as the electrical hydrocyclones, magnetic hy- drocyclones, magnetic fluids hydrocyclones, electrochemical hy- drocyclones, and hydrocyclones enhanced by flotation;(8) More investigations on effects of various flocculants on separation- performance parameters; apart from flocculant-assisted hydro- cyclones, more novel hydrocyclones with the help of other che- mical or biological methods should be developed;(9) Mechanism behind the hydrocyclone separation enhanced by ad- justing back pressure;(10) Development of more intelligent hydrocyclones, which can verse the amendment of operating state of hydrocyclones with corresponding change in operating state (e.g., underflow dis-charge pattern), with advanced artificial intelligence technologies;(11) Effects of the interaction between various parameters (e.g., the feed pressure and the hydrocyclone diameter) on separation-per- formance parameters of hydrocyclones by employing the response surface methodology, multi-criteria decision analysis, and ortho- gonal experimental methodology; integrative application of en- hanced-separation hydrocyclone technologies developed by opti- mizing operating parameters and conditions and those developed by optimizing geometric parameters, particularly the technologies with low energy consumption and small split ratio; drawing les- sons from enhanced-separation technologies of gas cyclones [5], vortex tubes [186], and other similar separation devices with strong swirling flow [187]; combination of hydrocyclone separa- tion with other separation approaches, such as the membraneseparation and nanofiltration;(12) Development of novel enhanced separation technologies of hy-drocyclones by optimizing operating parameters and conditions by using and developing i) advanced technologies, such as 3-D printing technology [188], x-ray computed tomography (x-ray CT) [170], high-speed imaging techniques [189], inline measurement techniques of droplet and particle size distribution, the three-di- mensional three-component velocimetry technique (i.e., volu- metric three-component velocimetry technique) [190], and the four-dimensional three-component velocimetry technique (in- corporating the time dimension); ii) more effective and affordable CFD techniques including the DPM model [118,191], CFD coupled with the discrete element method (DEM) (CFD-DEM) [192,193], coarse-grained CFD-DEM model [194], two-fluid model [195], and dimensionless rotation parameter which establishes a cri- terion for choosing cost-effective and accurate turbulence models [196]; and iii) modern particle processing techniques through which particles with the uniform parameters (e.g., shape, size, and density) can be manufactured;(13) Enlargement of the application scope of hydrocyclones, especially in the disposal of solid waste, wastewater, and waste gas in pet- rochemical, HVAC (heating, ventilation, and air conditioning), nuclear, aviation, aerospace, and other heavy industries.