Group model-building here refers to a system dynamics model-building process in which a client group is deeply involved in the process of model construction. The problem that is modelled can be reasonably well defined, but it can also take the form of an ill-defined or messy problem, i.e., a situation in which opinions in a management team differ considerably. These messy managerial situations are difficult to handle, primarily because thus far little theoretical work has been conducted to shed more light on the question why these messy situations exist and why it may be difficult for a management team to reach agreement. This article fills this theoretical gap by drawing on literature from sociology, (social) psychology and small-group research. Insights from this literature are discussed and translated into guidelines for conducting Group Model-Building projects for messy problems. The article ends with the conclusion that system dynamicists should include Group Model-Building and facilitation training in their teaching programs. Copyright © 1999 John Wiley & Sons, Ltd.