The importance of brassinosteroids (BRs, a specific class of ecdysone-like plant steroids) as essential endogenous regulators of growth and development is demonstrated through a growing number of well characterised Arabidopsis, pea, and tomato mutants deficient in BR biosynthesis or BR response. Thus, a rapid advancement in understanding the molecular genetics of BR biosynthesis and mode of action can be witnessed, which will be further enhanced through the availability of a set of extremely valuable molecular tools for the analysis of the biological function of BRs.