Cancer describes a class of diseases characterized by the uncontrolled growth of aberrant cells. Cancers kill by the destructive invasion of normal organs through direct extension and spread to distant sites via the blood, lymph, or serosal surfaces. The abnormal clinical behaviour of cancer cells is often mirrored by biologic aberrations such as genetic mutations, chromosomal translocations, expression of fetal or other discordant ontologic characteristics, and the inappropriate secretion of hormones or enzymes. All cancers invade or metastasize but each specific type has unique biologic and clinical features that must be appreciated for proper diagnosis, treatment and study. About 1.2 million new cases of invasive cancer are diagnosed each year in the United States, and about 500,000 people die annually of the disease. Cancer is the second most deadly disease and is expected to surpass heart disease early in the twenty-first century to top that vicious list. Over the past half century, the frequency of most cancers has been stable, but some dramatic changes have taken place. Steady declines in stomach and uterine cancer have occurred, the latter undoubtedly due to routine cytologic screening for cervical cancer. The cause of the decline in stomach cancer is unknown. The most striking change has been the increases in lung cancer in both men and women, undoubtedly related to smoking.