Rejection of tissue- and organ-grafts is due to the existence of tissue- or transplantation-antigens which are present in principle on every nucleated body-cell. These tissue antigens represent the phenotypical expression of a complex system of genes which are localized on an autosomal chromosome: the histocompatibility system. These systems may usually be compared with bloodgroup systems. Besides the successful use of tissue typing in transplantation, it became apparent that histocompatibility systems may have another function. Among others, tissue typing may be correlated with resistance to and susceptibility to particular diseases. However, it is not clear so far whether tissue antigens play a role themselves or only serve as genetic markers in this regulation of the immune response. The two possibilities are introduced, together with some instances. Finally, on the basis of possible veterinary and economic importance, studies on histocompatibility systems in pets and farm animals are reviewed, a number of possible uses being suggested.