Underventilated enclosure fires represent one of the largest causes of fire fatalities, and understanding their behavior is of great interest. The newest major release of the Fire Dynamics Simulator (FDS) has made significant progress towards providing a tool for accurate modeling of underventilated fire behavior. The extinction model provides a more accurate representation of the fire behavior in the compartment, but oxygen and temperature results are not satisfactory for severely underventilated fires. Using data from free burn calorimeter tests as input to FDS for fuel items in a compartment where the heat feedback can influence the pyrolysis can be a significant source of non-conservative error and give a poor representation of the burning behavior when ventilation is limited. This must be considered when choosing the heat release rate input to FDS, and the validity of calorimeter data should be evaluated for each case. The CO production model in FDS gives values lower than recorded in the fire tests due to uncertainty about the material data input and simplifications in the model. (Author Abstract)