The following sections will outline the heat diffusion theory underlying lock-in thermography experiments. First, in Sect. 4.1, the effects of the heat conduction on the surrounding of the sample will be discussed, presenting the definitions of a thermally thin and a thermally thick sample, and of the quasi-adiabatic state of a measurement. In Sect. 4.2, a simple method is being introduced of compensating the temperature drift in the initial heating-up phase of lock-in thermography experiments. These considerations should also be of interest when interpreting non-destructive testing experiments. Then, the following two sections will review the theory of the propagation of thermal waves for different heat source geometries. Based on these results, in Sect. 4.5 follows a summary of the most important relations for the quantitative interpretation of lock-in thermography measurements in terms of power sources for simple heat source geometries. In Sect. 4.5.1, the image integration/proportionality method is being introduced, which allows a quantitative interpretation of lock-in thermography results also for an arbitrary distribution of heat sources. Finally, Sect. 4.5.2 describes recent advances in the software-based correction of the effect of the lateral heat conduction within the sample on lock-in thermograms, implying also a quantitative interpretation of lock-in thermography results.