Brassinosteroids are endogenous plant hormones essential for the normal regulation of multiple physiological processes including cell elongation and division, vascular differentiation, vegetative and reproductive development, and numerous responses to the environment. Since the discovery of the chemical structure of brassinolide more than 35 years ago, extensive research on the mechanisms of BR action has helped define the BR biosynthetic pathway and identify the basic molecular components of BR signal transduction. This article provides historical context of studies from 1970 through 2005 involving chemistry, molecular genetics, and physiology that have provided the foundation for further mechanistic advances describing pathways and regulatory networks through which BR influences many aspects of plant growth and development.