Due to the wide variability of tissue interactions and the possibility of specific applications, the neodymium-yttrium-aluminum-garnet (Nd:YAG) laser is the most important surgical laser. With the adequate choice of application mode and relationship between interaction time and power density, it can be used for precise cutting in the contact mode with the bare fiber, with either a wide or small coagulation seam. With a handpiece a precise focal coagulation for preparation and hemostatic purposes is possible, as well as cutting with the focussed noncontact beam where additionally a wide coagulation seam is necessary. Endoscopically guided coagulation, vaporization, and cutting are possible with bare fibers which can be introduced through nearly all endoscopes. With increasing importance, the possibility for wide and homogeneous volume coagulation is used to destroy diseased tissues either by noncontact irradiation or interstitial placement of the fiber. Thus, the field of laser application in tumor therapy ranges from the treatment of superficial tumors to endoscopic tumor ablation, resection of neoplastic tissue in parenchymatous organs, and interstitial thermotherapy with coagulation of deep-seated primary and secondary malignancies. With its different application modes the laser can be used as a surgical instrument or as a central therapeutical method, whereby perfect control of tissue interactions is always possible by using either visual control for superficial and endoscopic procedures or magnetic resonance imaging and color-coded duplex sonography as a control for interstitial procedures. During 12 years of clinical work we have developed several application modes and have proved the Nd:YAG laser to be an effective instrument in tumor therapy.