While the majority of flowering plant species are hermaphroditic, gender di­ morphism, or the occurrence of two sexual morphs, has, nevertheless, evolved on repeated occasions. Gender dimorphism is found in nearly half of all angio­ sperm families and in approximately 10% of flowering plant species. Where plants are dimorphic in gender, they can also be dimorphic in secondary sex characters. We refer to dimorphism of the latter kind as sexual dimorphism, in keeping with the term's usage by most zoologists. This book is about the evolution of both forms of dimorphism -hence the book's lengthy title. Gender dimorphism in plants has been an active topic of research from theoretical and empirical perspectives, and has been the focus of several re­ cent reviews and book chapters. By contrast, sexual dimorphism in plants is of the much less widely appreciated. Indeed, the last comprehensive review subject dates back to Lloyd and Webb's 1977 paper on "Secondary Sex Char­ we first spoke of editing a book on sexual acters in Plants. " In addition, when dimorphism in plants, some people doubted that there was enough material to justify the effort. We hope that this book not only provides an update to Lloyd and Webb's seminal work but also dispels doubts about the widespread nature of sexual dimorphism in plants. We decided to combine reviews of both gender and sexual dimorphism in a single book, because each form of dimorphism can provide the evolutionary impetus for the other.