Despite documented benefits, the processes described in the new product development literature often prove difficult to follow in practice. A principal source of such difficulties is the phenomenon of fire fighting the unplanned allocation of resources to fix problems discovered late in a product's development cycle. While it has been widely criticized, fire fighting is a common occurrence in many product development organizations. To understand both its existence and persistence, in this article I develop a formal model of fire fighting in a multi-project development environment. The major contributions of this analysis are to suggest that: (1) fire fighting can be a self-reinforcing phenomenon; and (2) multi-project development systems are far more susceptible to this dynamic than is currently appreciated. These insights suggest that many of the current methods for aggregate resource and product portfolio planning, while necessary, are not sufficient to prevent fire fighting and the consequent low performance. / MIT Center for Innovation in Product Development under NSF Cooperative Agreement Number EEC-9529140, the Harley-Davidson Motor Company and the Ford Motor Company