ACM's first classification system for the computing field was published in 1964. Then, in 1982, the ACM published an entirely new system. New versions based on the 1982 system followed, in 1983, 1987, 1991, and now 1998. Users wanting to see how the terms in the 1991 system compare to those in the 1998 system can do so via the links below. (Note: double asterisks ** indicate terms retired as of January 1998.) The 1964 system is included here for historical reference.

• the 1998 system
• the 1991 system
• the 1964 system