Anorexia nervosa (AN), a condition of severe undernutrition, is associated with low bone mineral density (BMD) in adults and adolescents. Whereas adult women with AN have an uncoupling of bone turnover markers with increased bone resorption and decreased bone formation markers, adolescents with AN have decreased bone turnover overall. Possible contributors to low BMD in AN include hypoestrogenism and hypoandrogenism, undernutrition with decreased lean body mass, and hypercortisolemia. IGF-I, a known bone trophic factor, is reduced despite elevated growth hormone (GH) levels, leading to an acquired GH resistant state. Elevated ghrelin and peptide YY levels may also contribute to impaired bone metabolism. Weight recovery is associated with recovery of BMD but this is often partial, and long-term and sustained weight recovery may be necessary before significant improvements are observed. Anti-resorptive therapies have been studied in AN with conflicting results. Oral estrogen does not increase BMD or prevent bone loss in AN. The combination of bone anabolic and anti-resorptive therapy (rhIGF-I with oral estrogen), however, did result in a significant increase in BMD in a study of adult women with AN. A better understanding of the pathophysiology of low BMD in AN, and development of effective therapeutic strategies is critical. This is particularly so for adolescents, who are in the process of accruing peak bone mass, and in whom a failure to attain peak bone mass may occur in AN in addition to loss of established bone.