Osteoporosis is mainly diagnosed by means of bone densitometry. Dual X-ray absorptiometry examinations represent the basis for a highly reproducible and correct measurement. At present, densitometry is the only method at our disposal capable of assessing material-related fracture risk. The calculation of general fracture risk is dependent on a number of varios factors and is, therefore, not to be deduced from bone density values only. Reference values are necessary in order to estimate bone strength. The most sensible way to achieve this is to compare measured values with a normal, healthy population (T score). Material-related fracture risk increases with the decrease of bone density.