The genetic and molecular mechanisms that underlie the formation of angiosperm flowers have been studied extensively for nearly three decades. This work has led to detailed insights into the gene regulatory networks that control this vital developmental process in plants. Here, we review some of the key findings in the field of flower development and discuss open questions that must be addressed in order to obtain a more comprehensive understanding of flower formation. In particular, we focus on the specification of the different types of floral organs and on how the morphogenesis of these organs is controlled to give rise to mature flowers. Central to this process are the floral organ identity genes, which encode members of the family of MADS-domain transcription factors. We summarize what is currently known about the functions of these master regulators and discuss a working model for the molecular mechanism that may underlie their activities. This article is protected by copyright. All rights reserved.