Understanding heat transfer is vital in numerous applications in the field of power electronics. This thesis introduces some new reliable and efficient calculation methods for plate-fin heat sinks. There may be any number of electronic components attached to the base plate. The components may have arbitrary prescribed heat flux distribution. The state-of-the-art calculation methods found in the literature are based on conduction analysis, while the convective heat transfer is only treated as a boundary condition. This may lead to unphysical solutions. In this thesis, conjugated conduction and convection heat transfer problem is solved in the fins. However, the tedious solution of the Navier-Stokes equations is avoided by applying well-known analytical and experimental results for convective heat transfer. The conjugated heat transfer solution for the fins is used to determine the temperature field of the base plate. Some numerical examples are given to illustrate the fact that the present calculation methods give physically more realistic results than the methods found in the literature. The analysis in this thesis has been carried out assuming steady-state conditions. However, it is pointed out that the methods presented in the thesis can easily be generalised for the transient operation of the electronic components.