SummaryFloral traits often show correlated variation, both within and across species. One explanation for this pattern of floral integration is that different elements of floral phenotypes are controlled by the same genes, that is, that the genetic architecture is pleiotropic. Recent studies from a range of model systems suggest that the pleiotropy is common among the loci responsible for floral divergence. Moreover, the effects of allelic substitutions at these loci are overwhelmingly aligned with direction of divergence, suggesting that the nature of the pleiotropic effects was adaptive. Molecular genetic studies have revealed the functional basis for some of these effects, although much remains to be discovered with respect to the molecular, biochemical and developmental mechanisms underlying most quantitative trait loci (QTL) responsible for floral differences. Developing a detailed understanding of the nature of pleiotropic mutations and their phenotypic consequences is crucial for modeling how the genetic architecture of trait variation influences the tempo and trajectory of floral evolution. Contents Summary 80 I. Introduction 80 II. Extent and nature of pleiotropy in floral trait architecture 81 III. The evolution of pleiotropy 83 IV. Pleiotropy as an impediment or facilitator of phenotypic evolution 83 Acknowledgements 84 References 84