Notch signaling is a key regulator of cell-fate decisions in pre-natal skeletal development and is active during adult tissue renewal. In addition, its association with neoplasia suggests it is a candidate therapeutic target. We find attenuated Notch signaling enhances osteoclastogenesis and bone resorption, in vitro and in vivo, by a combination of molecular mechanisms. First, deletion of Notch 1, 2 and 3 in bone marrow macrophages (BMMs)1 directly promotes their commitment to the osteoclast phenotype. These osteoclast precursors proliferate more rapidly than do wild type (WT) in response to macrophage colony stimulating factor (M-CSF) and are sensitized to receptor activator of NF-B ligand (RANKL) and M-CSF, undergoing enhanced differentiation in response to low doses of either cytokine. Conforming with a role for Notch in this process, presentation of the Notch ligand, Jagged1, blunts the capacity of WT BMMs to become osteoclasts. Combined, these data establish that Notch suppresses osteoclastogenesis via ligand-mediated receptor activation. While Notch 1 and Notch 3 collaborate in regulating osteoclast formation, Notch 1 is the dominant paralog. In addition, Notch1 deficiency promotes osteoclastogenesis, indirectly, by enhancing the ability of osteoblast-lineage cells to stimulate osteoclastogenesis. This is achieved by decreasing the osteoprotegerin (OPG) / RANK ligand expression ratio. Thus, Notch1 acts as a net inhibitor of bone resorption, exerting its effect both directly, in osteoclast precursors, and indirectly, via osteoblast lineage cells. These observations raise caution that therapeutic inhibition of Notch signaling may adversely accelerate bone loss in man.