Abstract Early diagnosis is the key to the prevention and treatment of osteoporosis. A healthy skeleton has intrinsic properties that confer strength to resist fracture under ordinary stress. Some of the properties that confer strength and fracture resistance include: bone mass or density and bone quality determined by skeletal composition, fine structure and spatial organization, geometric properties, and rate of remodeling. The current approach to early diagnosis of osteoporosis is based on the measurement of bone mass or bone mineral density (BMD). Low bone mass is the single most accurate predictor of increased fracture risk. BMD accounts for 70% to 80% of the future fracture risk in older white women and is a far better predictor of osteoporosis than hypertension is for stroke or total cholesterol is for cardiovascular events in men. Perhaps in the future a better understanding and quantification of bone quality will help refine our ability to identify patients at risk. BMD can be measured at a variety of skeletal sites using several different methods that have been approved by the FDA. The basic attributes of each method will be addressed in this paper, with particular attention given to the method that is currently considered the gold standard, dual energy x-ray absorptiometry. The indications for BMD testing, clinical utility of BMD, frequency of follow-up testing, correlation between the available densitometry methods, problems and pitfalls in interpretation, and features of a satisfactory densitometry report of results will all be addressed. The current role of biochemical markers of bone turnover in the diagnosis and monitoring of treatment will be discussed briefly.