In constructing a general purpose programming language, a key issue is providing a sufficient set of data types and associated operations in a manner that permits both natural problem-oriented notation and very efficient implementation. The language EL1 contains a number of features specifically designed to simultaneously satisfy both requirements. The resulting treatment of data types includes provision for programmer-defined data types and generic routines, programmer control over type conversion, and very flexible data type behavior, in a context that allows efficient compiled code and very compact data representation.