Hydrocyclone is an important industrial solids-liquid separation equipment. Although widely used nowadays, the selection and design of hydrocyclones are still empirical and experience based. Although quite a few hydrocyclone models had been developed over the years, the validity of these models for practical applications was still not clear to all users. In this work, seven hydrocyclone models were evaluated. They are the more theoretically oriented models by Bohnet, Braun, and Mueller, semi-empirical models by Schubert/Neese and Svarovsky, plus the empirical models by Plitt and Krebs Engineers. Plant operation data from the Dow Chemical Company were used to compare with the predictions from these models. It was found that most of the models studied work well for certain cases but none of the models can predict all applications. The best results are obtained by using more than one model for predictions. Some experimental data are very important in choosing the models.