Basic helix-loop-helix (bHLH) transcription factors (TFs) belong to a family of transcriptional regulators present in three eukaryotic kingdoms. Many different functions have been identified for these proteins in animals, including the control of cell proliferation and development of specific cell lineages. Their mechanism for controlling gene transcription often involves homodimerization or heterodimerization. In plants, little is known about the bHLH family, but we have determined that there are 133 bHLH genes in Arabidopsis thaliana and have confirmed that at least 113 of them are expressed. The AtbHLH genes constitute one of the largest families of transcription factors in A. thaliana with significantly more members than are found in most animal species and about an equivalent number to those in vertebrates. Comparisons with animal sequences suggest that the majority of plant bHLH genes have evolved from the ancestral group B class of bHLH genes. By studying the AtbHLH genes collectively, twelve subfamilies have been identified. Within each of these main groups, there are conserved amino acid sequence motifs outside the DNA binding domain. Potential gene redundancy among members of smaller subgroups has been analyzed, and the resulting information is presented to provide a simplified visual interpretation of the gene family, identifying related genes that are likely to share similar functions. Based on the current characterization of a limited number of plant bHLH proteins, we predict that this family of TFs has a range of different roles in plant cell and tissue development as well as plant metabolism.