Hepatocyte nuclear factors (HNFs) play a key role in the establishment and maintenance of hepatocyte differentiation. Not only do HNFs control the expression of functional liver-specific genes, but they are also involved in regulating cell proliferation, morphogenesis, and detoxification in the liver. The review considers the main biological properties of HNF4α, the central regulator of hepatocyte differentiation; the patterns of its expression during embryo development and in adult tissues; the mechanisms regulating its activity and transcriptional properties; and the essential target genes. Studies of gene expression with rodent experimental models of carcinogenesis and clinical samples of human hepatic tumors revealed that transcriptional repression of HNF4α is associated with progression and dedifferentiation of tumors of this type. The possibility of a reversion of the dedifferentiated malignant phenotype by means of exogenous HNF4α expression confirms the important role of HNF4α in coordinating cell proliferation, differentiation, and the maintenance of epithelial morphology in epithelial cells of several types.