Knowledge of the temperature distribution in piston-cylinder assemblies is desirable for equilibrium studies and experiments under transient conditions. The accurate determination of equilibrium properties needs a homogeneous temperature within the sample and transient experiments often require a defined thermal gradient. Knowledge of the temperature difference between the sample and thermocouple is another important constraint for quantitative experiments. To this end, the temperature distribution within various different piston-cylinder assembly designs was modeled with a specially designed 3D-Finite-Difference (3D-FD) program and compared to laboratory observations. For the 3D-FD simulation, the temperature and pressure dependence of the thermal properties of piston-cylinder-assembly materials (NaCl, CaF2, pyrophyllite, Au, graphite, NiCr-alloy) was considered. Furthermore, the T-dependent resistivity of graphite was used to model the local heat generation of the graphite heater. Experimentally determined and modeled temperature distributions are in good agreement. This indicates that the 3D-FD program is useful and an appropriate tool in the design of virtual piston-cylinder assemblies to be used in homogeneous temperature-distribution or defined thermal gradient experiments. The influences of temperature, pressure, assembly design, assembly materials (CaF2, NaCl), stepped versus straight wall heater, and presence versus absence of gold capsules on the temperature distribution within piston-cylinder assemblies are modeled and discussed. Different piston-cylinder configurations are presented, optimized for equilibrium studies and transient experiments, focusing on low and predefined T-gradients, respectively.