Heat based tumor ablation methods such as radiofrequency (RF) and microwave (MW) ablation are increasingly accepted treatment methods for tumors not treatable by traditional surgery. Typically, an interstitial applicator is introduced under imaging guidance into the tumor, and tissue is destroyed by heating to above approximately 50 degrees C, with maximum tissue temperatures over 100 degrees C. Since high thermal gradients occur during the procedure, thermal conduction contributes significantly towards tissue heating. We created finite element method (FEM) computer models of RF and MW applicators, and determined the thermal conduction term, the resistive (for RF) or dielectric (for MW) loss term, and perfusion term. We integrated these terms over the heating period to obtain relative contribution towards tissue temperature rise (in degrees C) as a function of distance from the applicator. We performed simulations without and with perfusion, where perfusion was assumed to stop above 50 degrees C. During the first 6 minutes, direct heating by RF and MW were dominating throughout the tissue. Over the treatment period (12 min for RF, and 6 min for MW), thermal conduction was dominating at distances between than 12 and 19 mm from the RF electrode, while for MW ablation direct heating dominated everywhere. Even though thermal conduction significantly contributes towards tissue heating during ablative therapies, direct heating by RF or MW is dominating throughout most of the tissue volume. Tissue cooling due to perfusion is more significant during RF heating, in part due to the longer treatment times.