The development of Prolog IV, a new constraint logic programming (CLP) language, brings with it significant improvements in power and performance compared to its predecessor Prolog III. The richness of the constraint language enables Prolog IV to be applied to a very wide range of problems including scheduling or industrial planning applications, financial forecasting, or complex modelling in physics and chemistry. CLP languages have become increasingly popular because they require a high level of formalization of a problem, enabling it to be used by a wider audience and as a training tool to teach the resolution of difficult problems. CLP technology also provides a high level of tools, allowing users to concentrate on the formalization and programming of a problem, rather than low level implementation details such as memory management. Of the CLP languages, Prolog IV has the advantage of being theoretically sound, and, as it is inherited from Prolog, very simple. It will prove particularly useful as a prototyping tool, as the constraint approach is a very natural way of defining a problem, and the efficient algorithms used in Prolog IV will improve the prototype’s performance. Finally, a Prolog IV program is easy to install into a large software package as great care has been given to the interface between Prolog IV and other languages and systems.