Major wildland/urban interface fire losses, principally residences, continue to occur. Although the problem is not new, the specific mechanisms are not well known on how structures ignite in association with wildland fires. In response to the need for a better understanding of wildland/urban interface ignition mechanisms and a method of assessing the ignition risk, USDA Forest Service Fire Research is developing the Structure Ignition Assessment Model (SIAM). SIAM uses an analytical approach that relates the potential for sustained structure ignitions to the location and characteristics of adjacent fires and the structure's materials and design. SIAM's ignition risk assessment is based on a worst case estimate of the direct effect of flames leading to ignitions as well as ignitions from burning embers (firebrands). Initial SIAM results indicate that the flames of burning vegetation are not greatly effective in creating sustained ignitions. This suggests that firebrands and adjacent burning structures are significant causes of structure ignitions. Current experimentation is directed toward verifying these SIAM results.