Coronary vessel development is a crucial part of heart development requiring the interplay of the epicardial, myocardial and endocardial layers of the heart for proper formation. Coronary vascularization is regulated by a host of transcription factors further regulated by chromatin remodeling enzymes, including Histone Deacetylases (HDACs). To investigate the functions of HDACs in coronary vascular development, we have deleted Hdac3 in endocardial cells using Cre LoxP technology. Endocardial cell-­‐specific deletion of Hdac3 results in aberrant coronary vessel formation and complete postnatal lethality. We have thus shown that Hdac3 is a critical regulator of the coronary vascular development pathway.