Brassinosteroids (BRs) are critical plant hormones that regulate plant growth, development and plant responses to various environmental stresses. Molecular genetic studies in the past decades have determined and characterized both BR biosynthesis and signaling pathways. In the past several years, the field has made significant progress by using new technologies and through collaborations with other scientific disciplines. New BR signaling components have been discovered and the mechanisms of BR perception and signal transduction have been depicted at the molecular and in a few cases at the atomic levels. BR’s functional mechanisms in many growth and developmental processes have been studied. At the same time, genomics, proteomics and systems biology approaches have begun to reveal complex regulatory networks through which BRs regulate thousands of genes and crosstalk with other signaling pathways in the regulation of plant growth and responses to both abiotic (such as drought, cold, heat) and biotic (bacterial, fungal and viral) stresses. While much of the knowledge has been obtained with model plant Arabidopsis, BR’s function and signaling components have also been identified in important crops such as maize, rice and tomato. These studies have promised great potential of BRs in improving crop yield and plant performance under stress conditions.