The final clinical outcome of the osteoporotic process is a fracture, which can occur as a result of minimal trauma or even spontaneously. At present low bone mass is regarded as the main contributor to bone fragility, but possible qualitative changes in the bone matrix must also be considered. Two factors which determine the level of bone mass at any age are the obtained peak bone mass and duration and rate of bone loss. Peak bone mass is achieved during the first three decades of life. Genetic and nutritional factors as well as mechanical stress on the skeleton obviously play crucial roles in determining peak bone mass. Two phases of bone loss--age-related and menopause-related--dictate the final bone mass at old age. Postmenopausal osteoporosis is a particular example of unbalanced bone resorption leading to net bone loss. An increasing number of systemic and local factors have been found to participate in the regulation of bone remodeling.