For at least two decades, expansion of low-density residential development at the wildland-urban interface has been widely recognized as a primary factor influencing the management of US national forests. We estimate the location, extent, and trends in expansion of the wildland-urban interface (WUI) in the continental United States. We mapped the WUI by determining the intersection of housing density classes computed from refined US Census data with a map of wildfire hazards based on broad forest types using definitions of WUI from the Federal Register. Our methods allowed us to provide a more spatially precise estimation of the WUI that better reflects development patterns of interest to forest land managers. We defined three wildfire hazard classes based on vegetation type. ''High'' severity applies to vegetation types in which stand-replacing fires dominate both historical and recent fire regimes, e.g., lodgepole pine forest. ''Low'' severity applies where fuels and climate foster mostly low-intensity fires, e.g., aspen-birch forest. ''High (historically low or variable)'' applies to vegetation types in which fires historically were of low or variable intensity, but recently have often burned at high intensity because of a century of fire exclusion, e.g., southwestern ponderosa pine forest. In 2000, the WUI that includes a 3.2 km community protection zone occupied 465,614 km2, and contained over 12.5 million housing units. This is an expansion of over 52% from 1970, and by 2030 the WUI is likely to expand to at least 513,670 km2 with the greatest expansion occurring in the intermountain west states. Roughly 89% of the WUI is privately owned land and about 65% of the WUI occurs in high or high (historically low or variable) severity fire regime classes. © 2007 Elsevier B.V. All rights reserved.