Populus has many attributes of a model system in forest tree biology. Its reproductive biology among other aspects of genetics and breeding, has been studied from both the fundamental and applied point of view, largely due to interesting biological characteristics, such as anemogamoth pollination, dioecy and the opportunity for interspecific hybridization. The biology of both female and male components will be reviewed, with particular emphasis on pollen viability. pollen storage, female receptivity and length of seed maturation period. Breeding can be carried out in the greenhouse: classical pollination techniques are now improved with easy-to-use pollell viability tests and the routine use of embryo culture. However, some interspecific crosses are still hindered by incompatibility barriers. On a more basic level, pollen/style interactions have bees recently investigated by biochemical, physiological and cytochemical techniques. These studies point out that no general rules obtain for the rejection of the foreign pollen tubes, which seem to depend on the species involved. Rejection can occur in different floral tissues, the stigma, the stylodium or the ovule. Interspecific incompatibility has also been reported to be post-zygotic in the cross P. nigra (female) x P. deltoides (male). Attempts to overcome these different barriers have been successful, such as with mentor pollen. Due to the great variability of results with these empirical systems, other techniques are being developed and will be described, such as in vitro and in vivo fertilization of isolated flowers.