Cancer is characterized by progressive, persistent, perverted (abnormal), purposeless and uncontrolled proliferation of tissues. Special characteristics of cancer cells are uncontrolled proliferation, invasiveness, metastasis, and dedifferentiation. Additionally cancer cells enter a resting phase, only to multiply and recur later. There are four phases of cell cycle, i.e. G1, S, G2, and M. Cell-cycle specific drugs are mainly acts on dividing cells. Most anticancer agents act on rapidly multiplying cells (both cancer cells and normal cells). Hence, normal multiplying cells of bone marrow, gonads, epithelial cells of skin and mucous membrane are affected. Anticancer agents are classified into alkylating agents, antimetabolites, natural products, hormones and antagonists, and biological response modifiers, which are discussed briefly in this chapter. General principles of cancer treatment are also provided in this chapter.