Wildland urban interface (WUI): the area where homes and other development meet or intermingle with undeveloped wildland vegetation. A remote forest with no buildings is wildland, not interface, because no development is present. A dense downtown served by hydrants is urban, not interface.

Why the WUI is the heart of the fire problem: wildland fuels can carry fire to homes, and the homes themselves add fuel to the fire. People, structures and flammable vegetation sit side by side.

Embers are the hidden danger. The California Legislature found (Gov. Code §51175(a)) that the wildfire front is not the only source of risk: embers, or firebrands, travel far beyond the area impacted by the front and can ignite a structure or fuel on a site for a longer time. A home can burn even when the flame front never reaches it.

That is why the rest of this chapter looks at reducing risk around the structure itself: what fuels surround it, how it is built, and where and how development is allowed.