The design of power electronics modules and power packages is heavily influenced by thermal concerns. New substrate materials, thinner and thermally more conductive attachment materials are used to decrease the thermal resistance of a given module. When a new material or technology is applied, its reliability has to be tested thoroughly before the module can be considered for production. The reliability or the expected lifetime of the module can be expressed by a number of temperature or power cycles the system can withstand. These numbers however make more sense if they can be linked to the foreseen lifetime of the application where the power module will be applied. In our changing industrial environment the design of power modules is not aimed at maximum lifetime anymore, but it is determined by the application itself. In this article we will summarize the steps which take the designer from the application requirements (mission profile) to the expected lifetime of the application considering the reliability of the power modules used.