The development of saturated linkage maps using transferable markers, restriction fragment length polymorphisms, and microsatellites has provided a foundation for fruit tree genetics and breeding. A Prunus reference map with 562 such markers is available, and a further set of 13 maps constructed with a subset of these markers has allowed genome comparison among seven Prunus diploid (x ؍ 8) species (almond, peach, apricot, cherry, Prunus ferganensis, Prunus davidiana, and Prunus cerasifera); marker colinearity was the rule with all of them. Preliminary results of the comparison between apple and Prunus maps suggest a high level of synteny between these two genera. Conserved genomic regions have also been detected between Prunus and Arabidopsis. By using the data from different linkage maps anchored with the reference Prunus map, it has been possible to establish, in a general map, the position of 28 major genes affecting agronomic characters found in different species. Markers tightly linked to the major genes responsible for the expression of important traits (disease͞ pest resistances, fruit͞nut quality, self-incompatibility, etc.) have been developed in apple and Prunus and are currently in use for marker-assisted selection in breeding programs. Quantitative character dissection using linkage maps and candidate gene approaches has already started. Genomic tools such as the Prunus physical map, large EST collections in both Prunus and Malus, and the establishment of the map position of high numbers of ESTs are required for a better understanding of the Rosaceae genome and to foster additional research and applications on fruit tree genetics. T he major temperate fruit tree crops, apple (Malus ϫ domestica), peach (Prunus persica), cherry (Prunus avium and Prunus cerasus), plum (Prunus domestica and Prunus salicina), apricot (Prunus armeniaca), almond (Prunus dulcis), pear (Pyrus communis), quince (Cydonia oblonga), and loquat (Eriobotrya japonica), belong to the Rosaceae family. This also includes some other important crops such as strawberry (Fragaria ϫ ananassa) and rose (Rosa spp.). Most of these species are woody perennials with a long intergeneration period due to their juvenile phase and large plant sizes, which make them poorly suited organisms for classical genetic analysis. On the other hand, fruit trees have some advantageous features such as a long life, the existence of efficient methods of vegetative reproduction, the possibility of making interspecific crosses (frequent at the congeneric level), and a small basic genome; e.g., wild strawberry (Fragaria vesca) has a haploid genome size of 164 Mbp (1), and peach has a haploid genome size of 290 Mbp (2). Until recently, only very limited information existed on the genetics of phenotypic characters of simple inheritance; only 31 major genes had been described in peach (3), the best characterized of the Prunus species, or three genes in almond (4).The breeding methods used in these species have undergone very few changes over the last 50 years, and th...