Most of Donald Knuth’s fundamental contributions are to the theoretical side of IT. He’s the author of The Art of Computer Programming, for example, and developed ways to analyze the complexity of algorithms. He’s been referred to as the father of the analysis of algorithms.
He also developed a new methodology of development called literate programming, shifting the very goal of writing code: explaining to other humans what you’re telling the computer rather than just telling the computer. This method became the bases for Knuth’s system, WEB.
Knuth’s work has earned him various awards, including the Turing Award in 1974.