It is generally accepted that explosions of most impact-sensitive materials are thermal in origin. The mechanism of impact initiation presumably involves the formation of small regions of high temperature within the reactant. These “hot spots” may serve as nuclei from which grow the more gross phenomena—decomposition, deflagration and detonation. The phenomenon of impact explosion may be treated as a two-step process. First, the hot spot is formed in the explosive; second, exothermic reaction within the hot spot leads to sustained deflagration and/or detonation. Exothermic reaction is considered first, and critical hot spot size is determined by computer integration as a function of hot spot and ambient temperatures and the properties of the reactant. It is assumed in the analysis that the sample is infinitely large, heat flow is one dimensional, reactant consumption and heat losses are negligible, and exothermicity and all physical properties are constant. The validity of the last four assumptions is examined and generally established. The closed form equation for critical hot spot size is applicable when the hot spot temperature is between 400° and 9000°K and when the activation energy for decomposition lies between 5 and 60 kcal/mole. Based on several assumptions regarding the mechanism of hot spot formation, an impact sensitivity correlation is derived, presented graphically, and used to predict the sensitivites of 16 explosives, propellants, and recently synthesized organic compounds. The correlation predicts experimental sensitivities to 40% and indicates the degree to which impact sensitivity is affected by the several thermal and kinetic properties of explosive materials.