The disruption of homeostatic mechanisms that regulate normal cell growth and proliferation is a hallmark of cancer. Experimentally, many of the same genetic changes that lead to abnormal cell proliferation conspire to confer replicative immortality upon cells in culture. Correspondingly, several lines of evidence implicate cellular immortalization as a prerequisite for cell transformation. Recently much progress has been made in elucidating the cellular machinery that regulates cell lifespan. This review summarizes these recent advances in our understanding of these molecular mechanisms that contribute to human cell immortalization and transformation.