Percutaneous image-guided ablative therapies using thermal energy sources such as radiofrequency, microwave, high intensity focused ultrasound, and laser have received much recent attention as minimally-invasive strategies for the treatment of focal malignant disease. Potential benefits of these techniques include the ability to ablate tumor in non- surgical candidates, reduced morbidity as compared to surgery, and the potential to perform the procedure on an outpatient basis. This manuscript proposes a unified framework using the `Bioheat equation' for discussing aspects of thermal ablation therapies as they relate to the treatment of focal malignancies. Briefly, these include an understanding of under which conditions heat induces cellular damage, the types of energy sources which can supply this heat, and tissue properties such as perfusion mediated cooling which modify tissue response to the heat deposited. Additionally, the various facets of diagnostic imaging which are required to direct thermal ablation therapy are discussed. These include modalities which can be used for targeting of the lesion to be treated, determination of an optimal treatment plan, and assessment of results at long term follow-up.