Novel agents, including the proteasome inhibitor bortezomib, have significantly improved the response and survival of patients with multiple myeloma over the last decade. Despite these advances, many patients relapse or do not benefit from the currently available therapies; thus, multiple myeloma remains an incurable disease. Deacetylase inhibitors (DACi), including panobinostat and vorinostat, have recently emerged as novel agents being evaluated in the treatment of multiple myeloma. Deacetylases are a group of enzymes with effects on various intracellular proteins, including histones, transcription factors, and molecular chaperones. Although DACi inhibit cell growth and induce apoptosis in multiple myeloma cells as a single agent, synergistic activity has been observed when they were used in combination with bortezomib. The mechanistic basis of synergy is multifactorial and includes disruption of protein degradation and inhibition of the interaction of multiple myeloma cells with the tumor microenvironment. This review summarizes recent advancements in the understanding of the mechanism of action of proteasome inhibitors and DACi in multiple myeloma and examines the biological basis of their synergistic effects. Data from the studies summarized here have been used as the rationale for the implementation of phase II and III clinical trials of DACi, alone and combined with bortezomib, in relapsed and refractory multiple myeloma. Mol Cancer Ther; 10(11); 2034–42. ©2011 AACR.