This chapter discussed the causes and development of cancer based on different theories of mutational changes. These changes included the influences of stress, genetics, and environmental exposures as causes of cancer formation. The chapter also covered how chemotherapy works and the effects that chemotherapy has on both normal and abnormal cells. A summary of different major classes of chemotherapy and individual agents was included. Considerations for both safely providing chemotherapy and monitoring clients for adverse effects were discussed. Supportive therapies necessary when administering chemotherapeutic agents were also included.