Efforts to investigate the progression of events that lead normal human cells in culture to become neoplastic in response to carcinogenic agents have been aided by the development of the suitable in vitro model systems. For initial human cell transformation studies, a flat, nontumorigenic clonal line, originally derived from a human osteosarcoma (HOS), was used. When treated with chemical carcinogens such as N-methyl-N-nitro-N-nitrosoguanidine (MNNG) and 3-methyl-cholanthrene (3MC), the HOS cells underwent morphological alterations and acquired tumorigenic properties. These cell lines were very useful inasmuch as a non-ras cellular transforming gene, met, and an activated H-ras oncogene have been isolated from MNNG-transformed and 3MC-transformed HOS lines, respectively, by DNA transfection procedure. Alteration of p53 gene in chemically transformed HOS cell lines has recently been shown. Although carcinogens cause human cancer, normal human cells in culture have proven difficult to achieve. Neoplastic transformation of human cells in culture has recently been achieved by a stepwise fashion-immortalization and conversion of the immortalized cells to tumorigenic cells. One of the critical initial events in the progression of normal human cells to tumor cells is the escape from cellular senescence. With few exceptions, normal human cells require immortalization to provide a practical system for transformation studies. Thus, the role of carcinogenic agents in the development of human cancers is now being defined using a variety of human cells. The neoplastic transformation in human cell cultures is reviewed. In doing so, this author attempts to put into perspective the history of human cell transformation by carcinogenic agents, and to discuss the current state of the art in transformation of human cells in culture; thus providing insight into the molecular and cellular mechanisms involved in the conversion of normal cells to a neoplastic state of growth.