The past few years have seen considerable advances in speciation research, but whether drift or adaptation is more likely to lead to genetic incompatibilities remains unknown. Some of the answers will probably come from not only studying incompatibilities between well-established species, but also from investigating incipient speciation events, to learn more about speciation as an evolutionary process. The genus Arabidopsis, which includes the widely used Arabidopsis thaliana, provides a useful set of model species for studying many aspects of population divergence. The genus contains both self-incompatible and incompatible species, providing a platform for studying the impact of mating system changes on genetic differentiation. Another important path to plant speciation is via formation of polyploids, and this can be investigated in the young allotetraploid species A. arenosa. Finally, there are many cases of intraspecific incompatibilities in A. thaliana, and recent progress has been made in discovering the genes underlying both F(1) and F(2) breakdown. In the near future, all these studies will be greatly empowered by complete genome sequences not only for all members of this relatively small genus, but also for many different individuals within each species.