Carl Sassenrath (born 1957 in California) is an architect of operating systems and computer languages. He brought multitasking to personal computers in 1985 with the creation of the Amiga Computer operating system kernel, and he is the designer of the REBOL computer language, REBOL/IOS collaboration environment, the Safeworlds AltME private messaging system, and other products. Carl was a Principal Engineer at Roku, Inc. until his retirement in November 2023.

Carl Sassenrath was born in 1957 to Charles and Carolyn Sassenrath in California. His father was a chemical engineer involved in research and development related to petroleum refining, paper production, and air pollution control systems.

In the late 1960s his family relocated from the San Francisco Bay Area to the small town of Eureka, California. From his early childhood Sassenrath was actively involved in electronics, amateur radio, photography, and filmmaking. When he was 13, Sassenrath began working for KEET, a PBS public broadcasting television station. A year later he became a cameraman for KVIQ (ABC affiliate then) and worked his way up to being technical director and director for news, commercials, and local programming.

In 1980 Sassenrath graduated from the University of California, Davis with a B.S. in EECS (electrical engineering and computer science). During his studies he became interested in operating systems, parallel processing, programming languages, and neurophysiology. He was a teaching assistant for graduate computer language courses and a research assistant in neuroscience and behavioral biology. His uncle, Dr. Julius Sassenrath, headed the educational psychology department at UC Davis, and his aunt, Dr. Ethel Sassenrath, was one of the original researchers of THC at the California National Primate Research Center.

During his final year at the university, Sassenrath joined Hewlett-Packard's Computer Systems Division as a member of the Multi-Programming Executive (MPE) file system design group for HP 3000 computers. His task was to implement a compiler for a new type of control language called Outqueue—a challenge because the language was both descriptive and procedural. A year later, Sassenrath became a member of the MPE-IV OS kernel team and later part of the HPE kernel group.

While at HP Sassenrath became interested in minimizing the high complexity found in most operating systems of that time and set out to formulate his own concepts of a microkernel-based OS. He proposed them to HP, but found the large company complacent to the "smaller OS" ideas.

In late 1981 and early 1982, Sassenrath took an academic leave to do atmospheric physics research for National Science Foundation at Amundsen–Scott South Pole Station. Upon returning, Sassenrath reached an agreement with HP to pursue independent research into new areas of computing, including graphical user interfaces and remote procedure call methods of distributed computing.

Later in 1981, impressed by the new computing ideas being published from Xerox PARC, Sassenrath formed an HP project to develop the modern style of window-based mouse-driven GUIs. The project, called Probus (for professional business workstation) was created on a prototype Sun Microsystems workstation borrowed from Andy Bechtolsheim while he was at Stanford University. Probus clearly demonstrated the power of graphical user interfaces, and the system also incorporated hyperlinks and early distributed computing concepts.