Fires in underventilated compartments remain an unresolved problem for fire brigades. While some brigades have specific guidance in place regarding procedures for approaching and fighting such fires, this guidance is, for the most part, based on anecdotal evidence, having been instigated following historical incidents involving fire-fighter injuries or fatalities. As fire-fighters open a door or window to an under-ventilated compartment fire, there is a risk of a sudden fire development that may come in the form of a rapid flare up or backdraught. A programme of reduced scale fire experiments was designed to obtain deeper insight into this phenomenon. The results of these experiments are presented showing the conditions under which changes in the ventilation conditions of an underventilated compartment were beneficial for firefighting activities and under which conditions this was detrimental. This paper provides further scientific understanding of the fire dynamics in underventilated compartments when changes in ventilation conditions occur. The use of ventilation tactics, preceded by strict safety precautions and comprehensive tactical considerations, appears to be a practical solution that reduces the likelihood and severity of a potential backdraught. The results presented could be used to develop simple guidance, which may be used in fire brigade practice, to decide when and how to intervene in this kind of fires.