This chapter discusses the principles of chemotherapy, and includes information on the mechanism of action of commonly available drugs (anthracyclines, Vinca alkaloids, taxenes, alkylating agents, bleomycin, platinum–based agents, antimetabolites, topoisomerase inhibitors, hormonal agents, antiandrogens, cell signalling inhibitors, and growth factor receptor inhibitors), cellular principles of chemotherapy, pharmacological principles of chemotherapy (effects of hepatic and renal impairment, pharmacogenetics, dose intensity, locoregional drug administration, increase of the dose per administration, and treatment intervals), principles of combination chemotherapy (activity as a single agent, different mechanisms of action and resistance, dose-limiting toxicities, and cell-cycle-related and biochemical interactions), and adjuvant and neoadjuvant systemic therapy.