Many mathematical models exist for calculating various features of wildland fire behavior. Some are easy to use, some very complicated, but all will be found to produce results which do not always agree with observed fire behavior. In some instances, the disagreement can be quite significant. There are three principal reasons for such disagreement, no matter which models are used: The model may not be applicable to the situation, the model's inherent accuracy may be at fault, and the data used in the model may be inaccurate. If one applies a model in a situation for which the model was not intended to be used, the 'error' in the model's prediction can be very large. Most models have the following kinds of limitations and should not be expected to predict what they do not pretend to represent: 1) the fuel bed modeled is continuous, uniform, and homogeneous. The more the real fuel situation departs from this ideal, the more erratic the prediction will be when compared to real fire behavior, 2) some models assume that the fuel bed is a single layer and is contiguous to the ground (e.g., Andrews et al. 2003); in other words, there is no distinct gap between fuel layers (e.g., a forest stand with ground/surface fuels and crown or aerial fuels), 3) fire spread by spotting (flying embers or firebrands) is not modeled by some models (e.g., Andrews et al. 2003), so fire spread in those situations where spotting is important will likely be poorly estimated, and 4) fire whirlwinds and similar extreme, fire-induced atmospheric disturbances are not modeled. Guidelines exist as to when such phenomena are expected, but actual predictions are not yet within the state-of-the-modeler's art.