Brassinosteroids (BRs) collectively refer to the steroidal plant hormones that stimulate dramatic growth by cells and organs. Because of their essential roles, mutants that are defective in either the biosynthetic or signaling pathway exhibit severe dwarfism. Numerous reports from biochemical, genetic, and physiological studies agree that DWARF4 mediates the flux-determining step in the biosynthetic pathway. Rather than exercising allosteric feed-back regulation of the enzyme by the end product brassinolide, transcriptional control of the DWF4 gene is considered the most important for precise modulation of the pool size for bioactive BRs. DWF4 expression is localized to actively growing tissues, including shoot and root tips, and the junction tissues between inflorescences and roots. This expression is significantly down-regulated by exogenous or endogenous applications of BRs, but increased by auxin treatment. Direct measurement of the endogenous BRs in different tissues of Arabidopsis has demonstrated that the accumulated BRs are mainly attributable to DWF4 expression. Regarding their interaction with other hormones, BRs induce many auxin-associated genes, e.g., Aux/IAA, GH3, and SAUR. Therefore, it is likely that the brassinosteroids synthesized by a limited number of tightly controlled cells in actively growing tissues are responsible for the demands by those tissues. Future research should elucidate which portion of the BRs is transported via which mechanisms, as well as reveal the roles of auxin-associated genes expressed in a BR-dependent manner.