The formation of flowers in the model plant Arabidopsis thaliana is one of the main systems to study the genetic mechanisms underlying the growth and development of angiosperms. Research conducted over the past 30 years led to the discovery of key regulatory genes for many of the processes that take place during flower morphogenesis. Although it is currently not always known how the products of these genes function at the molecular level, recent work has led, in many cases, to substantial progress in our understanding of their activities. In fact, knowledge obtained with Arabidopsis is now so plentiful that it can be used as a basis for a comparative analysis of flower development to unravel the genetic mechanisms underlying flower evolution and to explain the astounding variations in flower architecture that can be found amon the similar to 250,000 extant angiosperm species. In this chapter, we summarise the main findings and concepts relating to Arabidopsis flower development. We describe how technological progress has led to big steps forward in our understanding of how flowers are formed from a small number of undifferentiated stem cells, and we outline current gaps in our knowledge and discuss possible strategies that could be used to fill them in the future.