Finite calculation of divergent selfenergy diagrams

In the framework of causal perturbation theory, it is possible to avoid ultraviolet divergences in the calculation of Feynman diagrams from the start, making subsequent regularization of divergent diagrams unnecessary. We give a simple introduction to the most important calculational features of the method. The method is also applied to the ’divergent’ general massive two-loop sunrise selfenergy diagram, where it leads directly to an analytic expression for the imaginary part of the diagram, whereas the real part can be obtained by a single integral dispersion relation.