ABSTRACT High turbulent flow conditions exist in hydrocyclones. This turbulence cannot be disregarded and the separation takes place under the influence of the centrifugal field and the turbulent transport (eddy diffusion) in the two-phase flow. A separation model for the classification in dilute suspensions (Φp in feed < 0.05 - 0.1) was developed which results in the grade efficiency curve (separation curve, separation function). On its basis the cut size d50 and the sharpness of separation x = d75/d25 are defined. The suitability of the model was examined by means of experimental results of several authors. With the help of the model the role of the essential process variables can be quantitatively or at least qualitatively explained. Further improvements and developments are possible in connection with advances in the description of two-phase flows, in the correlation between design and turbulence parameters as well as regarding dense-flow separations.