Both animal and plant steroids mainly rely on transcriptional factors to bring about specific physiological responses; however, the signaling mechanisms that regulate these transcriptional factors are different. Steroid binding inside an animal cell directly alters the transcriptional activity of intracellular steroid receptors, whereas brassinosteroid (BR) binding to a cell surface receptor activates a phosphorylation-mediated signaling cascade that changes the amount, subcellular localization, and/or DNA-binding activity of a family of novel transcription factors. Genetic and molecular studies conducted over the past several years have uncovered several crucial BR signaling components that have dramatically increased our understanding of the signaling process of plant steroid hormones. In this review, we discuss the biochemical mechanisms of these signaling proteins for regulating the activity of the membrane BR receptor and the transduction of the BR signal from the cell surface to the nucleus.