The American Cryptogram Association (ACA) is an American non-profit organization devoted to the hobby of cryptography, with an emphasis on types of codes, ciphers, and cryptograms that can be solved either with pencil and paper, or with computers, but not computer-only systems.
The ACA was formed on September 1, 1929. Initially the primary interest was in monoalphabetic substitution ciphers (also known as "single alphabet" or "Aristocrat" puzzles), but this has since extended to dozens of different systems, such as Playfair, autokey, transposition, and Vigenère ciphers.
The association has a collection of books and articles on cryptography and related subjects in the library at Kent State University.
There is also a regular journal called "The Cryptogram", which first appeared in February, 1932, and has grown to a 28-page bimonthly periodical which includes articles and challenge ciphers.