Brinch Hansen made seminal contributions in the areas of operating systems, concurrent computing and parallel and distributed computing that were influential in shaping the development of those fields and advancing them from ad hoc techniques to systematic engineering disciplines. The influence of his work was amplified by his "relentless search for simplicity," his insistence on clarity, writing and rewriting his papers many times before publishing them, becoming "one of the best explainers in the business," and his commitment to testing concepts in working systems – Brinch Hansen implemented and distributed working systems for new concepts he developed, noting: Operating systems The RC 4000 multiprogramming system introduced the now-standard concept of an operating system kernel and the separation of mechanism and policy in operating system design. Modern microkernel architectures trace their roots to the extensible nucleus architecture of the RC 4000. Improving microkernel performance was a major theme in operating system research for three decades after the RC 4000. Brinch Hansen's text, Operating System Principles, was the first comprehensive textbook on operating systems. Eventually published in six languages (English, Japanese, German, Czech, Polish and Serbo-Croatian), it remained in print for decades, and years after the RC 4000 system it described had become outdated. In 1990, nearly two decades after its initial publication, P. J. Plauger reviewed it, saying: In 2005, Association for Computing Machinery (ACM) members voted Operating System Principles a top 25 classic book, in a survey to choose out-of-print classic books to add to the ACM Digital Library. Operating System Principles ranked 15th in the survey, appearing on 8.8% of ballots. It is now available from the ACM Digital Library. Using Concurrent Pascal, Brinch Hansen demonstrated that it was feasible to fully implement operating systems in high level languages, and that doing so reduced the development effort by one to two orders of magnitude. Furthermore, entire operating systems could be published, and understood in their entirety. Brinch Hansen did precisely that in The Architecture of Concurrent Programs, leading Roy Maddux and Harlan Mills to declare: Concurrent programming Brinch Hansen and Hoare developed the monitor concept. Brinch Hansen published the first monitor notation, adopting the class concept of Simula 67, and invented a queueing mechanism. Hoare refined the rules of process resumption. Brinch Hansen created the first implementation of monitors, in Concurrent Pascal. More than a dozen additional monitor-based languages had been created by 1990: Simone, Modula, CSP/k, CCNPascal, PLY,