Prototyping as an approach to systems development is currently widely discussed, although somewhat less widely adopted. One of the main reasons for this is that project managers are worried about how to control the iteration inherent in a prototyping approach to development. The paper describes a framework for controlling the process of prototyping that is based on the typical changes that arise through prototyping. This Change Classification Method for controlling prototyping has been adopted, monitored, and evaluated during the development of a commercial estimating system for the Royal Dockyards, UK. A brief description of this project is followed by an examination of the proposed method of control in operation. An acceptable method of control will enable project managers to feel more confident in introducing prototyping to assist in the production of computer systems.