Recent findings concerning the human Y chromosome are reviewed. Normally the presence of the Y chromosome is related to male differentiation; however until recently nothing was known about chromosome Y and the development of the testis at the molecular level. Initially the most plausible assumption in this field was the "H-Y hypothesis" which suggested that the presence of a male specific antigen called H-Y, controlled by a gene(s) on the Y chromosome, was responsible for the differentiation of the primitive gonad into a testis. An alternative was a series of DNA sequences termed Bkm which were thought to play an important role in sex determination. These two hypothesis were refuted. More recently, molecular studies in XX males have shown the presence of Y-DNA in their genome suggesting that a small fragment of the Y (probably a single gene) is responsible for testis development in the majority of these patients. This gene has been termed TDF or ZFY and encodes a protein that could act as a regulating factor of the male pathway. Also reviewed are the actions of various genes recently assigned to the Y chromosome.