Liver fibrosis is due to excessive deposition of extracellular matrix by fibroblasts that are actived in response to chronic liver injury; including viral, autoimmune, toxic and mechanical stresses These cells produce many of the constituents of the matrix, including type I, type II and type V collagen and fibronectin. However, the precise origin of these fibroblasts is controversial. Quiescent hepatic stellate cells are generally believed to be the main source of fibrogenic cells. but accumulating evidence suggests that they are not the sole culprits. It was recently proposed that hepatocytes may undergo an embryonic transdifferentiation process, known as epithelial-mesenchymal transition, that allows them to acquire a fibroblastic phenotype during liver fibrosis. Experimental observations underlying this controversial hypothesis are presented and the limits of current methodological approaches are discussed.