The preamble is a piece of literature that originated from the forward to the first edition in the Big Book of Alcoholics Anonymous. From that, an editor of the Grapevine (a magazine specializing in content about recovery) borrowed the text in order to explain the fundamentals of A.A. This new explanation then turned into The Preamble as we know it and is now read at the beginning of every A.A. meeting. In essence, the preamble is Alcoholics Anonymous’ mission statement and also serves as a warm welcome to those who are interested.
The AA Preamble Text
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AA Preamble in the Big Book

The Preamble as seen in the first edition written in 1939.
In the Big Book, the preamble is located on page xiv in the forward to the first edition. However you will not find the version that is currently used today in the Big Book due to its copyright by the Grapevine. With permission, the Grapevine allows A.A. and those involved to use The Preamble as needed.