Rather than reviewing the many studies of cost effectiveness in osteoporosis, this paper reviews the principles of cost-effectiveness analysis and the gaps in our knowledge that are required to improve such analyses in osteoporosis. These include more information on the cost of fractures and their consequences on health states, particularly on an international basis. New developments include the incorporation of adherence into models, the use of the FRAX tool to assess cost-effectiveness in individuals with any combination of risk factors for fracture, and the setting of intervention thresholds based on cost-effectiveness.