Abstract: Neoplasia is a literal translation of the Greek word `new growth`. Neoplastic cells are said to be transformed because they replicate continuously, heedless and in perfect asynchronism to the regulatory influences that govern normal cell growth. As a result, a fundamental characteristic of neoplasms is the loss of responsiveness to regular growth control resulting from failure of the usual mechanisms that control cellular proliferation and maturation. Moreover, this random growth continues even in the absence of the stimulus. In this study, the cellular, molecular and biochemical mechanisms of carcinogenesis are analysed. This entails a detailed discussion of the cell cycle and its fine regulation together with an explanation of the link between the cell cycle and induction of cancer. Proto-oncogenes and tumor-suppressor genes are also discussed. Finally, the genetic aspect of cancer is tackled.