Improved grain yield has been a major focus of crop breeding programs around the world. With the accomplishments of the Rice Genome Project, genes regulating several agronomically important traits related to grain yield, such as tiller number, grain number, grain size, and plant height, have recently been identified. Although these findings have not been enough to fully characterize the mechanisms that regulate each trait, these genes and knowledge of the molecular mechanisms involved provide a set of tools that can be combined to achieve tailor-made breeding suitable for various programs aimed at higher grain yield.