The transcription factor OCT3/4 (also known as POU5F1 and Oct4) is regarded as one of the key regulators of pluripotency. Expression in nonmalignant cells is restricted to the pluripotent cells in the embryo and the primordial germ cells that will pass pluripotency to future generations via the gametes. Although major progress has been made in successfully identifying other players in the pluripotency network by using high-throughput screening methods, the exact mechanisms involved in regulation of OCT3/4 in vivo remain largely to be elucidated. In human tumors, OCT3/4 is the most informative marker in germ cell tumors diagnostics and is expressed in the precursor lesions gonadoblastoma and carcinoma in situ, as well as in invasive embryonal carcinoma and seminomatous tumors. Currently, the application of OCT3/4 for screening in high-risk patient populations is the novel focus of study. This article reviews OCT3/4 expression in normal development and germ cell tumors. In addition, expression in adult tissues and nongerm cell tumor malignancies in relation to splice variants and pseudogenes is discussed. An overview of the upstream and downstream factors in the OCT3/4 pathway as well as the epigenetic regulation of the gene is summarized and the possible role in oncogenesis considered.