Lawrence Gordon Tesler (April 24, 1945 – February 16, 2020) was an American computer scientist who worked in the field of human–computer interaction. Tesler worked at Xerox PARC, Apple, Amazon, and Yahoo!.

While at PARC, Tesler's work included Smalltalk, the first dynamic object-oriented programming language, and Gypsy, the first word processor with a graphical user interface (GUI) for the Xerox Alto. During this, along with colleague Tim Mott, Tesler developed the idea of cut, copy, and paste functionality and the idea of modeless software. While at Apple, Tesler worked on the Apple Lisa and the Apple Newton, and helped to develop Object Pascal and its use in application programming toolkits including MacApp.

Tesler was born on April 24, 1945, in The Bronx in New York City, to Jewish parents Isidore, an anesthesiologist, and Muriel (née Krechmer). Tesler lived in the Bronx through his childhood and graduated from the Bronx High School of Science in 1961. While in high school, he was guided towards computers by a teacher after showing the teacher an algorithm for generating prime numbers. Through this, he learned of a program at Columbia University where he was able to spend a half-hour each week on their computer systems, through which he taught himself programming before college. He went on to Stanford University in 1961 when he was 16, studying computer science and graduating in 1965 with a degree in mathematics. At Stanford, he had spent time as a student programmer for Joshua Lederberg on the LINC platform, and was a colleague of Larry Breed, Charles Brenner, Douglas Hofstadter, Roger Moore, and Bill Strachan.

During college and afterward, Tesler did some programming jobs on the side, and after graduation, worked as a consultant offering his programming services in the area. As he was one of only a few computer programmers listed in the Palo Alto phone directory he received a good deal of work. However, a regional recession caused this consulting work to dry up. Tesler also worked at Stanford Artificial Intelligence Laboratory (SAIL) in the late 1960s. With Horace Enea he designed Compel, an early single assignment language. This functional programming language was intended to make concurrent processing more natural and was used to introduce programming concepts to beginners.

During his time at Stanford, Tesler had participated in the counterculture of the 1960s, including the anti-Vietnam War protests. In the late 1960s, Tesler became involved in the Midpeninsula Free University, part of the Free Speech Movement, where he taught classes with titles such as "How to end the IBM Monopoly", "Computers Now", and "Procrastination".

Tesler left Stanford Artificial Intelligence Laboratory due to a number of factors in the early 1970s; he recognized that artificial intelligence would not be a usable technology for many years. Tesler is quoted as stating around this time that "AI is whatever hasn't been done yet.", with Douglas Hofstadter calling this Tesler's Theorem. Tesler says that he was misquoted, his actual statement being "Intelligence is whatever machines haven't done yet."

At this time, Tesler's marriage to his college girlfriend had ended in divorce. He took his daughter and moved to Oregon with a number of Vietnam War veterans who were returning there to build homes. There was little computing technology in this area and he could not get a job with the local bank, the only firm nearby with a computer system. He called Stanford to see if they had anything, and learned that Alan Kay, with whom Tesler had worked while at SAIL and then a member of Xerox Palo Alto Research Center (PARC), had been actively looking for him shortly after his departure. Kay wanted Tesler to join him at PARC. Tesler could not be hired at PARC due to a hiring freeze, so Tesler instead took a short-term project offered by Les Earnest from SAIL to write a "document compiler", a means to easily produce printable manuals from simple text files. In order to carry out this project, Tesler wrote Pub, which was then recognized as one of the first uses of markup language; it was later distributed on ARPANet.

PARC approached Tesler with an offer in 1971, but wanted to place Tesler in the On-Line Office System Group. Tesler had been more interested in Xerox's work with personal computers, so he turned down the offer. By early 1973, PARC had established development of the Xerox Alto, the first computer system designed around a graphical user interface (GUI), and Tesler accepted an offer for a position splitting his time between the Office System Group and the Learning Research Group, which Kay was heading. Some of Tesler's main projects at PARC were the Gypsy word processor for the Office System Group, and Smalltalk, the first dynamic object-oriented programming language, with the Learning Research Group. While working on Gypsy, Tesler and his colleague Tim Mott started writing ideas down envisioning the future of interactive computer use, considering current text-based user interfaces would move to GUIs with icons representing documents, and to develop ease-of-use. From there, the two developed the basic copy and paste function, now a standard feature in computing. Tesler also established the idea that computer interfaces should be modeless, where all actions are available to a user at all times, rather than modal, requiring the user to enter a specific mode to perform them. Gypsy was programmed to include both of these concepts.