The development and severity of inflammatory bowel diseases and other chronic inflammatory conditions can be influenced by host genetic and environmental factors, including signals derived from commensal bacteria(1-6). However, the mechanisms that integrate these diverse cues remain undefined. Here we demonstrate that mice with an intestinal epithelial cell (IEC)-specific deletion of the epigenome-modifying enzyme histone deacetylase 3 (HDAC3(Delta IEC) mice) exhibited extensive dysregulation of IEC-intrinsic gene expression, including decreased basal expression of genes associated with antimicrobial defence. Critically, conventionally housed HDAC3(Delta IEC) mice demonstrated loss of Paneth cells, impaired IEC function and alterations in the composition of intestinal commensal bacteria. In addition, HDAC3(Delta IEC) mice showed significantly increased susceptibility to intestinal damage and inflammation, indicating that epithelial expression of HDAC3 has a central role in maintaining intestinal homeostasis. Re-derivation of HDAC3(Delta IEC) mice into germ-free conditions revealed that dysregulated IEC gene expression, Paneth cell homeostasis and intestinal barrier function were largely restored in the absence of commensal bacteria. Although the specific mechanisms through which IEC-intrinsic HDAC3 expression regulates these complex phenotypes remain to be determined, these data indicate that HDAC3 is a critical factor that integrates commensal-bacteria-derived signals to calibrate epithelial cell responses required to establish normal host-commensal relationships and maintain intestinal homeostasis.