Three ingredients are essential for a wildland fire to start and to burn. First, there must be burnable fuel available. Then enough heat must be applied to the fuel to raise its temperature to the ignition point. And finally, there must be enough air to supply oxygen needed to keep the combustion process going and thus maintain the heat supply for ignition of unburned fuel. These three indispensable ingredients-fuel, heat, and oxygen-make up the fire triangle. All must be present if there is to be fire. In the following discussion, we will examine some of the basic characteristics of the heat segment of the fire triangle-the nature of heat itself. The level of difficulty of the treatment of topics in these publications varies, as signaled by the color of the cover: the blue cover group is generally elementary and the yellow cover group is intermediate.