To “spread like wildfire” is a phrase used to describe something that propagates unexpectedly, rapidly, and incessantly. Much of the unpredictable behavior of a wildfire stems from processes including the heat released from the combustion zone (flames), the structure and condition of fuels, the wind field and turbulence driving the fire, and terrain. However, the chemical make-up of the biomass fuel that powers a wildfire also provides a source for the capricious nature of combustion and the behavior of wildfires. This chapter provides a brief overview of the chemistry of biomass fuels and the chemical processes by which such fuels combust and release the energy that enables the fire to become self-sustaining. It then looks in some detail at the mechanisms through which the combustion chemistry driving the heat release from the fuels is influenced by the environment surrounding the combustion zone. In the worst instances these mechanisms can result in fire behavior that causes widespread death and destruction often over a very short period of time. In the best instances they enable fire to be used as a reliable tool for reducing the hazard present in the wild landscapes of our countryside and surrounding our homes.