Bone metastases are a process originally proposed as the “seed and soil theory” in the eighteenth century. Tumor cell disseminating from patients with breast or prostate cancer typically use the bony environment to grow outside the primary tumor location. The severe clinical consequences of bone metastasis such as pain, fractures, and hypercalcemia result from a serious misbalance of bone turnover. Most bone metastases cause catabolic changes of bone turnover. The severity of bone resorption is associated with tumor growth, suggesting the existence of a vicious cycle that needs to be interrupted. Osteoblastic metastasis showing signs of osteosclerotic lesions are observed in prostate cancer. Understanding the pathophysiology of bone metastases and their detrimental consequence provide the scientific basis for therapeutic interventions at various levels including homing of tumors to bone, survival and growth of the tumor cell in the bone niche, and the mechanisms causing bone destruction.