As an illustration of the enigma that cancer has presented to physicians and scientists over the years, it was not until the 1920s that meaningful attempts were made to define cancer. In the ensuing half-century, a number of definitions of this biological phenomenon were proposed, mostly by physicians and scientists. To confuse the field further, cUnicians, scientists, and lay persons have used such terms as cancer, neoplasm, tumor, and malignancy as if they were synonymous in every way. As a definition "A neoplasm is a heritably altered, relatively autonomous growth of tissue". This definition encompasses several concepts. Heritably altered indicates alterations that are transmitted to all progeny in a heritable and irreversible manner. Autonomy indicates that a cancer is not subject to the "rules and regulations" that govern the individual cells and the overall cell interactions of the functional organism. The adverb relatively modifies autonomous to indicate that neoplasms are not completely autonomous. In many instances the autonomy that a neoplasm possesses may be quite subtle and relative only to the tissue from which it arose. The term growth may indicate the rate of cell division as modified by the rate of cell death (apoptosis) or the rate of intracellular processes involved in the synthesis of macromolecules for use within the cell or for excretion by the cell. The final component of the definition is the term tissue, which stems from the fact that, at our present state of knowledge, cancer or neoplasia can be defined only in a multicellular organism. In this sense cancer may be called "the curse of evolution." The definition of neoplasia proposed by Ewing in the mid1930s was that of a pathologist knowledgeable in the biology of cancer as expressed in vivo. Today, the in vivo system is still the basic reference for our definition of the neoplastic cell. On the other hand, with the recent advances of carcinogenesis in vitro, characteristics of the cancer cell as it grows in tissue culture have been described. Although it is not yet possible to define the malignant cell in vitro without reference to its behavior in vivo, an ultimate goal of