The AS/400 is an IBM minicomputer for small business and departmental users, released in 1988 and still in production in February 2002, under the name iSeries 400.

Features include a menu-driven interface, multi-user support, terminals (IBM 5250) that are (in the grand IBM tradition) incompatible with anything else including the IBM 3270 series, and an extensive library-based operating system, OS/400.

The machine was originally based on a custom IBM CISC CPU, but was later migrated to a PowerPC based RISC CPU family eventually known as RS64. The latest models are based on the POWER4 processor.

The machine survives because its API layer allows the operating system and application programs to take advantage of advances in hardware without recompilation and which means that a complete system that costs $9000 runs the exact same operating system and software as a $2 million system. There is a 64-bit RISC processor operating system implementation.

Programming languages include RPG, assembly language, C, Java, COBOL, SQL, BASIC, and REXX. Several CASE tools are available: Synon, AS/SET, Lansa.

It was designed as the successor of the IBM System/38

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