Congregations include worship groups identified as churches, mosques, synagogues, temples or by some other name.

The estimated number of congregations varies and determining the precise number is impossible.  Many are small and may lack a permanent location, significant income or staff. Thus, they have little or no reason to incorporate or provide other documentation that would make them known to government agencies that track organizations. 

The standard source for estimates is the American Church Lists, a company that uses phone listings and other sources to provide comprehensive marketing data.  In 2010, ACL reported 297,000 congregations, down from 304,000 the previous year.

Other estimates have ranged as high as 370,000.  NCCS has adopted a middle-ground estimate of 327,000 congregations in its work on the Hudson Institute's Global Philanthropy Index.

See the work of Mark Chaves on the American Congregation Survey for additional insights.