General considerations regarding the relationship between rarity and endangerment of tree species are presented and some important requirements for a sound appreciation of the actual status of threat are outlined. In spite of the rather scarce or totally absent knowledge of most of our rare tree species this article tries to classify the currently rare tree species in Switzerland according to the outlined requirements and estimate their current status of endangerment. Possible strategies for conservation measures are proposed. Finally, results of a first genetic survey using isozymes are presented for Sorbus torminalis and Sorbus domestica. The presented results show an astonishingly high genetic diversity for both species which is comparable to abundant species such as beech or oak. Vegetative propagation seems to be an important life history trait of both species. It is further demonstrated that a phenotypic selection of 80 to 90 plus-trees for a seed orchard may be an excellent tool to conserve a regional gene pool and to produce offsprings with a high genetic diversity.