This chapter focuses on the developmental evolution of flowers and flower organ identity. It reviews some of the most important insights that have been gained from research on the developmental evolution of flowers regarding the nature of organ identity, organ integration, and the origin of evolutionary novelties. The chapter begins with a discussion of the uniqueness of flowers and the evolution of phylogeny and flower characters in angiosperms. It then examines the genetics of canonical flower development, along with the developmental genetic architecture of the flower Bauplan. It also considers flower variation and the identities of novel flower organs, the origin of the bisexual flower developmental type, perianth evolution and the origin of petals, and the realization that additional organ identities can evolve after gene duplications.