The concept of Open GIS depends on precise definitions of data, operations and interfaces. This paper argues for the use of functional programming languages as specification and prototyping tools for Open GIS components. It shows how functional programming languages fulfill the key requirements for formal specification languages and allow for rapid prototyping in addition. So far, it has never been possible to integrate specification and prototyping in a single, easy to use environment. Most existing specification methods lack appropriate tools for checking and prototyping, while existing tools lack either sound semantics or usability or both. The paper discusses the role of specifications in GIS, requirements for specification languages, and presents the basics of algebraic specifications as well as of functional languages. It then describes how functional languages can be used for writing and executing algebraic specifications. A brief example of a GIS data type specification in a functional language is presented, showing how specifications serve to describe differences in the semantics of GIS operations. We conclude that functional languages have the potential to achieve a breakthrough in the problem of specifying interfaces of interoperable components for Open GIS.