Spontaneous heating is a process requiring a balance of these factors: a reactant (oxygen), a substance of vegetable or petroleum origin, and the environment in which the process occurs. In order to determine whether spontaneous combustion has occurred, the investigator must become familiar with substances subject to self-heating. While spontaneous heating can occur in practically all organic materials when conditions are ideal, normally, the oils, fats, metals, charcoal, coal, and the combination of chemicals such as sodium, potassium, ammonia, chlorine, oxygen, and hydrogen are affected in uncontrolled conditions. Especially harvested plants have an extremely high tendency to spontaneously heat and ignite. It is important that the investigator ascertain the nature of the material where the spontaneous ignition allegedly originated, the temperature at the time preceding the fire outbreak, and the humidity and the amount of oxygen available. The analytical fire investigator must ensure that a postulated sequence of events in a fire not only fits the observed condition but also follows the physical and chemical laws of fuels and oxidizes. Five references are given.