Publisher Summary Osteoporosis is characterized by increased risk of fracture due to changes in the “quality” of bone. To appreciate why bone becomes weaker or less resilient to fracture with age in both men and women and in individuals of different races, a general knowledge of bone development and age-dependent changes is necessary. It is noted that there are both age- and sex-dependent differences in bone properties and composition, some related to the rate at which bones develop in boys and girls, some related to the impact of genes on the X-chromosome which produce proteins important for bone development and/or metabolism and some due to the direct effect of sex steroids on bone cells. To appreciate the discrete differences between bone structure and composition in men and women this chapter reviews the basics of bone composition and organization and the mineralization process from the point of view of sexual dimorphism, where such differences between men and women are recognized. Emphasis is placed on those factors that contribute to bone strength; geometry, architecture, mineralization, the nature of the organic matrix and tissue heterogeneity.