Clinical and Experimental Endocrinology, Chronic Diseases and Metabolism, KU Leuven, Belgium The biologically active form of vitamin D3, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], plays a critical role in the regulation of mineral and bone homeostasis. 1,25(OH)2D3 binds to the vitamin D receptor (VDR), a ligand-activated transcription factor that controls gene expression in vitamin D-target tissues like the intestine, kidney, and bone. Vitamin D deficiency during development or VDR impairing mutations lead to conditions such as rickets, secondary hyperparathyroidism, and hypophosphatemia, emphasizing the importance of 1,25(OH)2D3 signaling in maintaining mineral balance and skeletal health. A major function of 1,25(OH)2D3 is to stimulate intestinal calcium absorption, thereby ensuring calcium supply to the bone for proper mineralization. Moreover, 1,25(OH)2D3 can directly affect bone homeostasis as several bone cell types express the VDR. Indeed, chondrocytes, osteoblasts, osteocytes as well as osteoclasts express the VDR. Numerous transgenic mouse models have been created that either have reduced or elevated VDR expression in specific bone cell types and these models allow to decipher the direct effects of 1,25(OH)2D3 on bone homeostasis. In the event of a positive calcium balance, wherein the absorbed calcium either equals or surpasses calcium losses through feces or urine, the expression of VDR in osteoblasts or osteoclasts appear to play a minor role in calcium homeostasis, as evidenced by the minimal phenotype observed in bone cell-specific Vdr knockdown models. However, during a negative calcium balance, induced by a low calcium diet or reduced intestinal VDR signaling, VDR expression in osteogenic cells contributes to the maintenance of normocalcemia at the expense of skeletal integrity. Indeed, higher circulating levels of 1,25(OH)2D3 lead to reduced bone mineralization and increased osteoblastic expression of receptor activator nuclear kappa B ligand (RANKL), which in turn results in elevated osteoclastic bone resorption. Nevertheless, even in case of a negative calcium balance, osteoclastic VDR expression exerts no influence on bone homeostasis.