Disruptions in cardiac development cause congenital heart disease, the most prevalent and deadly congenital malformation. Genetic and environmental factors are thought to contribute to these defects, however molecular mechanisms remain largely undefined. Recent work highlighted potential roles of chromatin- modifying enzymes in congenital heart disease pathogenesis. Histone deacetylases, a class of chromatin-modifying enzymes, have developmental importance and recognized roles in the mature heart. This thesis aimed to characterize functions of Hdac3 in cardiac development. We found loss of Hdac3 in the primary heart field causes precocious progenitor cell differentiation, resulting in hypoplastic ventricular walls, ventricular septal defect, and mid- gestational lethality. In primary heart field progenitors, Hdac3 interacts with, deacetylates, and functionally suppresses transcription factor Tbx5. Furthermore, a disease-associated Tbx5 mutation disrupts this interaction, rendering Tbx5 hyperacetylated and hyperactive. By contrast, deletion of Hdac3 in second heart field progenitors bypasses these defects, instead causing malformations in the outflow tract and semilunar valves, with lethality prior to birth. Affected semilunar valves and outflow tract vessels exhibit extracellular matrix and EndMT defects and activation of the Tgfβ1 signaling pathway. In normal second heart field development, Hdac3 represses Tgfβ1 transcription, independent of its deacetylase activity, by recruiting the PRC2 methyltransferase complex to methylate the Tgfβ1 promoter. Importantly, knockouts of Hdac3 in differentiated cardiac cells do not fully recapitulate the progenitor-specific knockout phenotypes. These results illustrate spatiotemporal roles of Hdac3, both deacetylase-dependent and deacetylase-independent, in cardiac development, suggesting that dysregulation of Hdac3 in cardiac progenitor cells could be a contributing factor in congenital heart disease pathogenesis.