The purpose of this guidance is to assist sponsors of drug and biological products for the treatment or prevention of rare diseases in conducting more efficient and successful drug development programs. Although the statutory requirements for marketing approval for drugs to treat rare and common diseases are the same and issues discussed in this guidance are encountered in other drug development programs, these issues are frequently more difficult to address in the context of a rare disease for which there is often limited medical and scientific knowledge, natural history data, and drug development experience.