Advanced Computer Techniques (ACT) was a computer software company most active from the early 1960s through the early 1990s that made software products, especially language compilers and related tools. It also engaged in information technology consulting, hosted service bureaus, and provided applications and services for behavioral health providers. ACT had two subsidiaries of note, InterACT and Creative Socio-Medics.

Both writer Katharine Davis Fishman, in her 1981 book The Computer Establishment, and computer science historian Martin Campbell-Kelly, in his 2003 volume From Airline Reservations to Sonic the Hedgehog: A History of the Software Industry, have considered ACT an exemplar of the independent, middle-sized software development firms of its era, and the Charles Babbage Institute at the University of Minnesota has also viewed the company's history as important.

Advanced Computer Techniques was founded in New York City in April 1962 by Charles P. Lecht. It had an initial capitalization of $800, one contract, and one employee. Lecht, in his late twenties at the time, was a mathematician and entrepreneur whose involvement with the computer industry dated back to the early 1950s, including stints at IBM and the MIT Lincoln Laboratory.

The new firm's first job was fixing a language compiler on the UNIVAC LARC computer, which was being used by the United States Navy. UNIVAC awarded a $100,000 contract for the work; Lecht hired some programmers and the company's first office was in former servant quarters atop the Plaza Hotel. The firm was one of 40–50 software companies started in the early 1960s, many of which would go on to be forgotten.

Working on compilers continued to be part of the company's early efforts; its first compiler, for the FORTRAN language, was developed in the mid-1960s. This was followed by a COBOL compiler later in that decade. As the 1960s went on, ACT built a customer list of established companies and developed a reputation for delivering quality work on schedule. In September 1964 the company leased regular office space, the first of several locations it would have during its lifetime, all of which were within greater Midtown Manhattan on or near Madison Avenue. In addition to UNIVAC, early customers for the firm's compiler work included IBM as well as Honeywell. The company also developed some of the System/360 utilities for IBM.

With few trained computer programmers available at the time, Lecht hired those with musical, linguistic, or mathematical backgrounds, finding them to be successful at this new activity. The firm also did other system software as well as scientific programming projects, including some for the defense industry, and then started doing commercial applications development for large companies such as Union Carbide, United Airlines, Hoffman-LaRoche, and Shell Oil. Lecht fostered a relaxed working environment where dress was informal and hours flexible. He instituted a series of weekly reports that all developers had to file detailing their progress; these were communicated to the client, on the theory that "a client can get angry at us, but [they] can't be more than one week angry at us because we told [them] exactly where we were."

Lecht was a colorful and flamboyant character with an idiosyncratic sense of style, who went around on a motorcycle and was described as a "showman" by colleagues, customers, and competitors alike. At one point his office and desk were completely covered by silver square tiles. ACT benefited from his flair for publicity: He, together with the company, was profiled in The New Yorker in 1967 and later in industry publications such as Datamation, which once referred to him as "One of computerdom's most flashy characters". Even decades later, a computing historian recalled Lecht as "one of the real characters in the industry, a very, very unusual man."

Lecht published several textbooks on programming covering different languages. ACT organized a series of seminars for the American Management Association on project management for developing computer applications. The seminars were organized into a 1967 book by Lecht, The Management of Computer Programming Projects, that was likely the first book ever published on the topic. The company also published A Guide for Software Documentation in 1969, compiled and edited by Dorothy Walsh, which was again one of the first of its kind and was cited by a number of other publications in the years to follow.