Interspecific compatibility and incompatibility have been examined through the genus Populus. General methods of manipulation have been developed to break the incompatibility barriers. These methods are described and a hypothesis is put forward to account for the results. This proposes that at least two factors are involved, one attached to the pollen (P) and one to the stigma (S), and that the interaction of these (PS) is critically involved in the total process. Implications for future plant breeding are discussed.