The development of immortalized cell culture systems has been a tremendous asset to basic research as well as to the pharmaceutical and biotechnology industries. These cell culture systems have facilitated advances in our knowledge of the biochemical and metabolic mechanisms underlying cell function and disease pathology. The immortalization of epithelial cells has been particularly important for elucidating specialized cell function and/or neoplastic progression. The strategies required for the generation of epithelial cell systems, in particular, human airway epithelial cells, can serve a paradigm for other specialized cell systems such as endothelial cells. Numerous airway epithelial cell systems have been generated from tumor tissue and primary cells. The methodologies for establishing these transformed and immortalized cultures have relied primarily on oncogenic viruses and expression vectors carrying oncogenes and/or telomerase. Because of the extensive work that has already been carried out over the last two decades, these human epithelial cell systems provide an ideal model for defining the parameters that lead to cell transformation and immortalization as well as those that result in cells with differentiated functions similar to those of primary epithelial cells. With this in mind, the strategies and techniques employed to develop transformed and immortalized cells that can be used to study neoplastic progression or differentiated function can be readily extrapolated to other cell systems.