Brassinosteriods (BRs) are polyhydroxylated triterpenoids essential for plant growth. BRs participate in a myriad of developmental processes, such as seed germination, pollen tube growth, male fertility, vascular development, owering time, and senescence. During the plant life cycle, BRs modulate the plant response to environmental factors such as light, temperature, salt, and pathogens among others (Bajguz and Hayat 2009). BRs were originally discovered in Brassica napus L. pollen (Grove 1979). e past two decades, genetic and biochemical analyses have identied the main BR signaling and synthesis components in the plant model species Arabidopsis and rice (Vert et al. 2005; Kim and Wang 2010; Li 2010). e BR pathway is currently among the most highly studied signal transduction pathways in plants (Vert et al. 2005; Kim and Wang 2010; Clouse 2011; Figure 17.1). In aerial plant organs, the growth-promoting properties of BRs are exemplied by their positive eect on cell elongation. Recently, the comprehensive characterization of BR contribution to root growth has shown that in addition to cell elongation, BR-mediated cell cycle progression is central for growth and meristem maintenance in the primary root. is chapter summarizes the state of knowledge on BR signaling in  Arabidopsis and the current understanding of BR action in root development.