Accreditation is an indication that a school has met minimum standards established by a standard-setting organization. The primary organization that establishes quality standards for business schools in North America is the AACSB - The International Association for Management Education. To become accredited by the AACSB, a business school must have been operating for at least five years, must have a critical mass of qualified faculty, and must show that its programs are in keeping with a written mission statement. Additionally, it must satisfy a variety of technical requirements. The majority of American business schools, about 800, are members of the AACSB. Of these, only about 300 are accredited and some are candidates for accreditation.
Other accrediting bodies exist that are not as well recognized as the AACSB, although they are legitimate and serve valid accreditation purposes.
Many fine schools exist that are not accredited, yet have excellent programs. They may be too young to be accredited, their program might be very specialized, they may not have reached a required size, they may be in a region not covered by a well-known agency, and so on. For this reason, the Official MBA Guide provides a large amount of data that can tell you something about the nature of a school and its programs. It is important to examine the schools' data carefully to determine whether they meet your needs.