Peter Landin was an important figure in the development of programming language theory. He was instrumental in creating the distinction between syntax and semantics that we know today. Further, he helped show how programming languages could be mapped onto the Lambda Calculus even after they were designed (as opposed to a language like LISP that was designed to be a Lambda Calculus). This of course has had tremendous impact on our understanding of functional programming and compilers. He died in 2009.