The last 15 years have seen the development of wildland and wildland-urban interface (WUI) fire behavior models that make use of modern numerical methods in atmospheric and combustion physics. Currently, these approaches are too computationally expensive for operational use and, as for any fire behavior model, require validation through comparison to full-scale measurements. However, these ‘physics-based’ models have the potential of providing a more complete understanding of fire behavior over a wider range of environmental conditions than empirically based models. The promise of physics-based models is not to replace the use of simpler and faster models, but to provide a well-founded understanding of the limitations of simpler models and a means of improving them. In this project the Wildland-Urban interface Fire Dynamics Simulator (WFDS) computer model suite was developed. The WFDS suite includes a physics-based fire model and a simple fire perimeter propagation model based on a level set method. The basic implementation of the level set fire perimeter model is equivalent to FARSITE in that they both provide a method for propagating a fire perimeter based on prescribed spread rates for the head, back, and flank fires. The WFDS-physics-based model produces a time evolving fireline that agrees well with AU grassland experiments. The WFDS-level set approach produces fire perimeters that are similar to FARSITE’s for simple scenarios. These findings support the use of the WFDS-physics-based model to evaluate the performance of the WFDS-level set model and, by surrogate, FARSITE. Fire propagation via the level set approach agrees with the physics-based results for surface fires with uniform grassland fuel on level ground. However, there are significant discrepancies between the physics-based and level set fire perimeters under scenarios of interacting fire lines, fire spread around a single fuel break, fire spread through multiple fuel breaks representing a WUI community, and spread up a slope. These discrepancies are consistent with the limitation of the level set model to fires evolving in a quasi-steady manner with constant head, flank, and back fire spread rates. Non-steady fire-atmosphere interactions, which resulted in non-constant spread rates, are present for cases in which the physics-based and level set models disagree. Given the growing interest in the development and application of WUI risk assessment methods that use FARSITE-type simple fire perimeter propagation models, it is important to identify the potential shortcomings of these models. Physics-based models are an approach to meet this goal. We do not claim, at this point, that we know how well the physics-based models will work in general. But we do believe that, since they include the driving physical processes, they offer a promising way to investigate and identify “watch-out” scenarios in which the simple fire perimeter propagation models are most likely to be in error. The WFDS model suite developed in this project provides a framework for conducting this investigation.