Thermal ablation techniques are experiencing application in many different fields of medicine. Recently, experimental studies have been performed by various authors concerned with dosimetry and laser-tissue interaction. In order to study the effects of interstitial laser energy on biological tissue, we examined different tissue models which compared important parameters during laser application. We have performed the following in vitro, in vivo and ex vivo studies by comparing a neodymium: YAG (1064 nm) and diode laser (830 nm) equipped with interstitial laser fibers. In vitro studies which examined the influence of changes in power and time duration of application were performed on potato, muscle, liver and kidney. In vivo studies (porcine model) also examined different power settings at designated time intervals. Ex vivo studies with isolated perfused kidney (IPK) investigated the effects of power, application time, perfusion pressure and different perfusion mediums (saline solution, anticoagulated blood). In vitro studies revealed necrotic lesions in all tissues. Although no power threshold could be obtained for liver tissue (early onset fiber damage), potato, kidney and muscle tissue demonstrated their own respective power threshold. Furthermore, when using the Nd:YAG laser, we observed that higher power settings had permitted a quicker necrosis induction, however within its own treatment power spectrum, the diode laser was capable of inducing larger lesions. In vivo studies demonstrated that early onset diffuser tip damage would prevent exact documentation of laser-tissue interaction at higher power levels. Results obtained with our standardized ex vivo model (IPK) revealed smaller necrotic lesions with saline than with blood perfusion and also demonstrated the important role which perfusion rate plays during laser-tissue interaction. We found that pigmented, well vascularized parenchymal organs with low stromal content (kidney, liver) and a higher absorption coefficient induced larger necrotic volumes than organs without these characteristics. Higher power settings demonstrated side effects, (e.g. popcorn effect or uncontrollable vaporization induced by extreme hyperthermia) in every animal tissue in all three trials. Our experimental interstitial laser studies have shown that many factors influence the size outcome of the necrotic lesion and that treatment parameters (treatment time, power setting) must be optimally combined to obtain a controlled and predictable necrotic lesion in certain tissues.