Oscillographic methods have been used to examine the periods between impact and explosion and impact and rebound in the explosion of several solid high explosives confined between steel surfaces and subjected to the blow of a falling weight. Impact-explosion times up to several hundred $\mu $sec. were observed, the time increasing with diminishing impact sensitiveness of the explosive and with lower impact energies. The impact-rebound time always exceeded the impact-explosion time in the same experiment. The effect of foreign substances on the times was examined; the time lag before explosion was diminished by adding quartz but could not be increased by the addition of soft materials. The mean velocity of blast-wave propagation from explosion was observed to increase with the impact-explosion time. The conditions for the growth of a hot spot in a volume of explosive are formulated in terms of thermal and kinetic data, and critical hot-spot temperatures computed for several explosives show a correlation with impact sensitiveness. The calculated time of hot-spot reaction is much less than the impact-explosion times. Viscous flow under high pressures is postulated as a possible mechanism of hot-spot formation, the impact-explosion period being mainly the time required for the pressure to rise to the necessary high value. A direct demonstration of the occurrence of high-flow velocities under impact was made.