We give an overview of the Glasgow Haskell compiler, focusing especially on way in which we have been able to exploit the rich theory of functional languages to give very practical improvements in the compiler. The compiler is portable, modular, generates good code, and is freely available. 1 Introduction Computer Science is both a scientific and an engineering discipline. As a scientific discipline, it seeks to establish generic principles and theories that can be used to explain or underpin a variety of particular applications. As an engineering discipline, it constructs substantial artefacts of software and hardware, sees where they fail and where they work, and develops new theory to underpin areas that are inadequately supported. (Milner [1991] eloquently argues for this dual approach in Computer Science. ) Functional programming is a research area that offers an unusually close interplay between these two aspects (Peyton Jones [1992b]). Theory often has immediate practical appl...