High-speed photographic techniques have been used to investigate the sympathetic detonation of solid explosives by shocks propagated across air gaps and solid barriers. It has been observed that initiation takes place within the body of the receptor stick, rather than at the surface, if the shock pressure is appreciably less than the detonation pressure. The depth in the receptor at which initiation occurs depends systematically upon the pressure of the incident shock; the lower the pressure the deeper the point of initiation. Detonation always occurs at the shock front, but, under the conditions of the experiments completed thus far, does not propagate backward into the preshocked explosive. The propagation velocity of the detonation wave in the receptor is, at least initially, greater than that observed under ordinary conditions. Studies of initiation by impact have shown many points of similarity. Initiation takes place within the body of the target explosive block, at a point ahead of the striking projectile, except at very high velocities of impact. The depth in the explosive and the distance ahead of the projectile at which initiation occurs depend mainly upon the velocity of the projectile and upon the shape of its front. In agreement with previous work, it has been shown that the kinetic energy of the impacting projectile is not a basic parameter in determining the probability of initiation or the conditions under which it occurs.