In slurry reactors, finely ground catalyst is suspended in a liquid phase. Compared to other types of reactors, this offers advantages in terms of temperature control and catalyst utilization. In addition to the chemical reaction, complex hydrodynamic and mass transport processes must be taken into account when designing the reactor. This is usually done via empirical correlations. This chapter provides an overview of the underlying physical relationships, on the basis of which the applicability of available correlations can be critically evaluated.