Founded by Justin Winsor, Charles Ammi Cutter, Samuel S. Green, James L. Whitney, Melvil Dewey (Melvil Dui), Fred B. Perkins, Charles Evans, and Thomas W. Bicknell on October 6, 1876, during the Centennial Exposition in Philadelphia and chartered in 1879 in Massachusetts, its head office is now in Chicago.
During the Centennial Exposition in Philadelphia in 1876, 103 librarians, 90 men and 13 women, responded to a call for a "Convention of Librarians" to be held October 4–6 at the Historical Society of Pennsylvania. At the end of the meeting, according to Ed Holley in his essay "ALA at 100," "the register was passed around for all to sign who wished to become charter members," making October 6, 1876, to be ALA's birthday. In attendance were 90 men and 13 women, among them Justin Winsor (Boston Public, Harvard), William Frederick Poole (Chicago Public, Newberry), Charles Ammi Cutter (Boston Athenaeum), Melvil Dewey, and Richard Rogers Bowker. Attendees came from as far west as Chicago and from England. The aim of the association, in that resolution, was "to enable librarians to do their present work more easily and at less expense." The association has worked throughout its history to define, extend, protect and advocate for equity of access to information.
ALA membership is open to any person or organization, though most of its members are libraries or librarians. Most members live and work in the United States, with international members comprising 3.5% of total membership.
The official purpose of the association is "to promote library service and librarianship." Members may join one or more of eleven membership divisions that deal with specialized topics such as academic, school, or public libraries, technical or reference services, and library administration. Members may also join any of seventeen round tables that are grouped around more specific interests and issues than the broader set of ALA divisions.
According to wikipedia