Spontaneous ignition has become more known and understood in the fire investigation community over the past 25 years, but is still missed by many seasoned experts. Fire, by definition, is a rapid oxidation process resulting in the evolution of light and heat in varying intensities, NFPA 921; 3.3.66. Oxidation is defined as a chemical reaction that is most commonly caused by atoms gaining oxygen. Therefore, fire investigators must have some knowledge of elementary chemistry to successfully complete origin & cause investigations. Spontaneous heating is the result of the slow oxidation of a combustible material, NFPA 921; 3.3.179. So, what exactly is oxidation?