Radiofrequency (RF) ablation is a treatment modality that kills unwanted tissue by heat. Starting with cardiac arrhythmia treatment in the 1980s, RF ablation has found clinical application in a number of diseases, and is now the treatment of choice for certain types of cardiac arrhythmia and certain cancers. During RF ablation, an electrode is inserted into or steered intravascularly to the target tissue region under medical imaging guidance. Then, a tissue volume surrounding the electrode is destroyed by heating via RF electric current. This paper reviews the biophysics of tissue heating during RF ablation. Effects of electrical tissue conductivity and its change with temperature are discussed. Procedures and devices specific for cancer treatment and for arrhythmia treatment are presented with a brief discussion of additional clinical applications.