To understand convection in terrestrial planets, numerical simulations are used. Convection can be described using the conservation equations for energy, momentum and mass, as well as the equations of state, and initial and boundary conditions. By coupling conservation equations for chemical species to that system, the interplay between convection and differentiation can be investigated. The solutions of these models depend on many parameters, which are not well known. A successful model should reproduce the observations e.g. provided by space missions. We distinguish among several types of models. In parameterized models the efficiency of heat transport is described by a scaling law. In this case, only a global energy budget is considered, which is computationally inexpensive. It allows the investigation of a wide parameter space and is useful for a global overview.