The Rosaceae are a family of approximately 95 genera that has been described as 'indefinable' because of the great morphological diversity that it exhibits. The family nevertheless is clearly monophyletic, based on recent studies of DNA sequence variation. The morphological diversity of the Rosaceae has been recognized taxonomically in classifications of the family that, after segregating the Chrysobalanaceae and Neuradaceae, have divided it into four subfamilies (sometimes recognized as families themselves), and into from eight to 18 tribes. Within these groups the number and composition of genera may also be a problem, especially where barriers to intergeneric hybridization are weak or absent. Also, the size of some genera may depend on the species concept employed, and the extent to which this concept takes into account the occurrence of apomixis and hybridization. One line of attack on problems like these has been to estimate phylogenies for the family as a whole, or for individual subfamilies, tribes, and some of the larger genera. Molecular data support recircumscription of the two largest subfamilies (Maloideae and Rosoideae), rejection of Amygdaloideae and Spiraeoideae because they are not monophyletic, and redefinition of many of the tribes. These studies offer insights not only about the monophyly of these groups and their relationships with each other, but also about their origin, as in the case of subfamily Maloideae. Other approaches involve investigating reproductive behavior and its consequences, or reevaluating morphological homologies using comparative and developmental data. All of these approaches will be required in order to understand the patterns of variation seen in the Rosaceae in terms of both the overall pattern of evolutionary diversification and the underlying processes that have shaped that diversification.

Key words: classification, evolution, phylogeny, Rosaceae