Bone formation and resorption (or bone turnover) are reflected by levels in blood and urine of various enzymes from bone cells, byproducts of bone matrix synthesis, or breakdown products of bone. These biochemical markers provide information about projected rates of bone loss and can be useful in assessing the effectiveness of antiresorptive therapy for osteoporosis. Biochemical markers provide adjunctive information to bone density measurements, and may be independent predictors of fracture risk. For clinical use, combining markers of resorption and formation should give better information than a single marker.