Bone morphogenetic proteins (BMPs) play a crucial role during all stages of embryonic development. Although only two major signaling pathways have been characterized (the p38 and Smad pathways), the BMP signaling is complex and includes several negative feedback mechanisms. This article reviews the current state of BMP receptor signaling and provides a summary of the crosstalk of the BMP receptor pathway with other major signaling pathways. (C) 2003 Elsevier Inc. All rights reserved.