In the previous chapter, the biology of malignancies and their caricature of normal tissue renewal suggested important principles that guide the development and use of treatment strategies. The current chapter presents how the treatment principles have been reduced to practice and how treatment principles relate to biological principles governing malignant growth. The chapter is organized around the four conventional treatment modalities available today: surgery, radiotherapy, cytotoxic chemotherapy and targeted therapy. (The term “conventional” refers to therapy accepted as the best available standard treatment.) Because of their different strategies, each modality is associated with specific risks and side effects, and this chapter builds a scientific understanding of the modalities' approved uses, successes, limitations, and toxicities. As explained in the preceding chapter, the goal of using the modalities is cytoreduction – hopefully the complete eradication of all cancer cells from the body. If not possible, then the goal becomes reducing the number of cancer cells in the body to the point that the time required for malignant stem cells to replace them is longer than the patient's life, giving rise to a cure. If sufficient cytoreduction is not achievable, then relapse ensues at some point in the future that depends on the amount of surviving malignant tissue and its rate of repopulation. The younger the patient at time of diagnosis, the more effective the treatment must be at eradicating malignant cells.