Abstract-The epicardium plays an essential role in coronary artery formation and myocardial development, but signals controlling the development and differentiation of this tissue are not well understood. To investigate the role of platelet-derived growth factor receptor (PDGFR)␤ in development of epicardial-derived vascular smooth muscle cells (VSMCs), we examined PDGFR␤ Ϫ/Ϫ and PDGFR␤ epicardial mutant hearts. We found that PDGFR␤ Ϫ/Ϫ hearts failed to form dominant coronary vessels on the ventral heart surface, had a thinned myocardium, and completely lacked coronary VSMCs (cVSMCs). This constellation of defects was consistent with a primary defect in the epicardium. To verify that these defects were specific to epicardial derivatives, we generated mice with an epicardial deletion of PDGFR␤ that resulted in reduced cVSMCs distal to the aorta. The regional absence of cVSMCs suggested that cVSMCs could arise from 2 sources, epicardial and nonepicardial, and that both were dependent on PDGFR␤. In the absence of PDGFR␤ signaling, epicardial cells adopted an irregular actin cytoskeleton, leading to aberrant migration of epicardial cells into the myocardium in vivo. In addition, PDGF receptor stimulation promoted epicardial cell migration, and PDGFR␤-driven phosphoinositide 3Ј-kinase signaling was critical for this process. Our data demonstrate that PDGFR␤ is required for the formation of 2 distinct cVSMC populations and that loss of PDGFR␤-PI3K signaling disrupts epicardial cell migration. , and past data have demonstrated that many coronary VSMCs (cVSMCs) are derived from the embryonic epicardium. 2,3 Whereas several genes have been identified that are essential for the formation, attachment, and spreading of the epicardium, few genes have been identified that are essential during epithelial-to-mesenchymal transition (EMT) and subsequent differentiation into cVSMCs and cardiac fibroblasts.Platelet-derived growth factor receptor (PDGFR) tyrosine kinases are 1 family of signaling proteins that are potentially involved in epicardial cell function. Analyses in the mouse have shown that PDGFR␤ signaling promotes proliferation and migration of VSMCs in multiple vascular beds including the heart. 4 -8 Therefore, we investigated the function of PDGFR␤ signaling during epicardial development. We have examined PDGFR␤ Ϫ/Ϫ , epicardial-specific PDGFR␤ mutant, and PDGFR␤ signaling-deficient embryos. We discovered that epicardial deletion resulted in the absence of cVSMCs distal to the aorta and that PDGFR␤ signaling through phosphoinositide 3Ј-kinase (PI3K) was required for proper cytoskeletal organization in epicardial cells. Our results designate PDGF receptor signaling as another growth factor system involved in epicardial development.