Editor's note: During the past few years land use and traffic models have been developed to the stage at which they are now actively being applied in professional practice. This event has met with both favor and disfavor, ranging from enthusiastic acceptance to unqualified rejection. We are confident that the extremes of these responses cannot long be sustained and that the models will find an appropriate and constructive place in city and metropolitan planning-a place acceptable to all or most members of the profession. We have therefore invited Britton Harris to pose the questions that the models raise. As a city planner who is himself a maker and user of models, but who is also highly respectful of the skills and accomplishments of those who reject them, he is well equipped to examine the issues. In this carefully reasoned and sympathetic essay, in which he makes his own position clear, he has succeeded, we think, in clearly explaining what a model is, in posing some of the important questions that the profession now faces, and in opening the floor for a deliberate and orderly consideration of these central questions. We are eager that the JOURNAL serve as the forum for these deliberations, and the readers are encouraged to participate.