Fire occurrence depends on the physical and biological factors of the fire environment, but fires don’t necessarily occur when the environment is primed. We describe a probability model that relates fire occurrence to the spatial/temporal history of fire in a region, in addition to characteristics of the fire environment that might serve as explanatory variables. The model expresses 1) the probability of having a fire at a particular place and day; 2) having a large fire (> 100 acres), given an ignition; and 3) the unconditional probability of having a large fire. The modeling methodology is described for a forested area in Oregon. The probability model is being developed as a tool for regional fire risk assessments.