Chemotherapy involves the use of cytotoxic drugs to treat cancer. The goals of chemotherapy are to cure cancer, improve survival rates, or relieve symptoms. Key principles of chemotherapy include: (1) using drug combinations to increase efficacy and decrease resistance, (2) treating micrometastatic disease early on, and (3) dose intensity being important for response. Adjuvant chemotherapy after surgery or radiation has improved survival rates for several cancers like breast cancer and osteosarcoma by targeting remaining micrometastatic disease.