There are many stages and a large investment of both time and money involved in the process of research and development before a new drug can be prescribed for clinical use. Of the thousands of new molecular entities, only one or two are approved for commercialization, after having endured a trajectory of 12 to 15 years in clinical trials in both animals and humans, demonstrating their therapeutic effectiveness and safety. There are three large administrators of medicines that control the process of new drug registration, the FDA - Food and Drug Administration of the USA being the largest and most well known. This article is based on their model and details the various stages that the molecule must undergo before finally being administered to patients. The future holds many exciting promises for new drug development with the advent of the human genome project and other highly advanced technological methods. However, the main challenge still remains, which is to guarantee the access of basic medicines to the majority of the world s population that is still without them (Rev Méd Chile 2001; 129: 99-106)