A theoretical examination is made of the ignition of explosives by a hot, stagnant gas pocket (hot spot), such as may be produced in a porous material by impact or shock. The effect of the various factors that control the surface heating and ignition event over a wide range of conditions (including reaction buildup) are discussed. At low pressures the heat flux and ignition characteristics are controlled by the gas properties (for fixed material kinetics). The computed ignition energies of several explosives at 3 msec ignition time are in general agreement with the low pressure gas compression measurements of Bryan and Noonan. On the other hand, under high (shock) pressure conditions, the gas pocket size and material thermal conductivity also affect the ignition characteristics, and the effect of hot spot size is dominant. It is shown that relatively large gas pockets cannot ignite the material in microsec time periods, The calculations thus provide a detailed semiquantitative explanation of the experimental observation that gaseous heating at adiabatically compressed voids is not the cause of initiation of a shocked explosive.