The definition of Osteoporosis, according to World Health Organization, is based on densitometric analysis of bone. The method most widely used to measure bone density is dual-energy X-ray absorptiometry, since it provides precise measures with minimum radiation. The difference between Osteoporosis and Osteopenia is currently considered to be quantitative. First, normal bone mass is defined as a value of bone mineral density (BMD) within one standard deviation with respect to young reference adults (when peak bone mass is achieved) of the same sex and race. Osteopenia, or low bone mas, is defined as a BMD value 1-2.5 standard deviations below the reference range. Osteoporosis is defined as a BMD value more than 2.5 standard deviations below the reference range. These data are presented as T scores, i.e. the number of standard deviations above or below reference values. Histologically, trabeculae appear thinner and there are fewer connections between them. This entity may appear with: 1) high turnover (greater increase of resorption than formation) secondary to estrogen deficit, heparin, hyperparathyroidism and anticalcineurins (cyclosporin); 2) low turnover (greater decrease of formation than resorption) secondary to hepatopathy or ageing; or 3) mixed (increase of resorption and decrease of formation) secondary to glucocorticoid treatment.