We studied experimentally the initiation ofPETN with a dispersity of 3700–22,000cm 2/kg and a density of 0.6–1.3g/cm 3 by a laser (λ=1.06 µm and τ=40nsec). Data processing was based on dimensional analysis of the process and study of the phenomena accompanying initiation (crater formation and sudden change in optical characteristics). This made it possible to describe empirically the complex dependence of the threshold initiation energy ofPETN on its density and dispersity, the irradiation-zone diameter, and the acoustic impedance of a transparent base plate. The mechanism of laser initiation ofPETN is considered.