As semiconductor devices continuously get both smaller andfaster, a profound analysis of the dynamic (time-dependent) thermal behaviouris becoming more and more important. In this article some basicconcepts and numerical methods for thermal characterization in the frequencydomain are explained. Two practical examples are given. Firsta simple model is presented for a small package mounted on a thin platesubject to convective cooling. The results are in good agreement with experimentalmeasurements. Next the influence of the finite heat propagationspeed is investigated. The calculated temperature distributions give a tasteof the thermal wave-like phenomena that are probably to occur in futurenano-scaled electronics.