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Osteoporosis is a general disorder of the skeleton characterised by a decrease in bone mass, with damage to the microarchitecture leading to an increase in bone fragility and fracture risk. The incidence of this illness will increase in the future because of the aging of the population and increasing risk factors. Many guidelines have been proposed by qualified authorities--those of the European Agency for the Evaluation of Medicinal Products (EMEA) being the latest published. The aim of treatment of the osteoporosis is to increase, maintain or improve bone mass as well as its strength, with a view to decreasing the incidence of bone fractures. With regard to preclinical studies, in vitro studies--such as those using osteoblast or osteoclast cultures--allow a better understanding of the mechanism of action of drug treatment. The evaluation of bone quality should be performed in two species, such as the ovariectomised female rat model and larger animals (ewe, sow, primate etc.). Phase I studies are designed to enable determinations of pharmacokinetic profiles and bone diffusion and to offer indications of the putative clinical relevance of the dosages. For phase II studies (double-blind controlled studies versus placebo, ideally with a duration of 24 or sometimes 12 months), tests of three dosages are recommended, and the bone mass is considered as a relevant substitution criterion. The aim of secondary osteoporosis prevention studies (randomised double-blind and comparative controlled design versus placebo) is to avoid the occurrence of new bone fractures, and the main evaluation criterion is the number of patients with new fractures. The study length should not be less than 3 years. For evaluation of primary osteoporosis prevention, efficacy in the prevention of bone fracture is the prerequisite--before the use of bone mass as the main evaluation criterion. This criterion can be evaluated by alterations in bone mineral density at the rachis level. Reference drugs such as estrogens can be an alternative to placebo comparison. The tolerability of drugs for the treatment of osteoporosis requires evaluation in long-term studies and the collection of postmarketing data. The International Conference on Harmonisation (ICH) could lead to uniform guidelines for the US, Europe and Japan.