Endochondral bone formation is a complex process that requires precise transcriptional control of gene expression programs in mesenchymal progenitor cells.  Histone deacetylases (Hdacs) coordinate transcription by enzymatically removing acetyl groups on histones and other proteins.  Hdac inhibitors are used clinically to manage mood disorders, cancers and other conditions, but are teratogenic to the developing skeleton and increase fracture risk in adults.  This presentation will:  1) review the function and expression of Hdacs in bone, 2) describe the effects of Hdac inhibitors on bone density in vitro and in vivo, and 3) explain the role of Hdac3, one of the proteins blocked by Hdac inhibitor therapies, in bone density and marrow adiposity during aging.