The plant-specific B3 superfamily encompasses well-characterized families, such as the auxin response factor (ARF) family and the LAV family, as well as less well understood families, such as RAV and REM. In Arabidopsis, there are 118 B3 genes, and in rice there are 91 B3 genes. The B3 domain is present in genes from gymnosperms, mosses and green algae, indicating that the B3 domain evolved on the plant lineage before multicellularity. The aim of this review is to phylogenetically characterize the members of the B3 family in Arabidopsis and rice and to review the function of the B3 genes that have been studied to date.