In this article, we review some recent studies in predicting impact sensitivity for different classes of energetic crystals based on first-principles band gap. Based on these investigations on metal azides, a first-principles band gap criterion is founded to measure impact sensitivity for a series of energetic crystals. For energetic crystals with similar structure or with similar thermal decomposition mechanism, the smaller the band gap is, the easier the electron transfers from the valence band to the conduction band, and the more they becomes decomposed and exploded. Applications of this criterion on other series of energetic crystals show that the first-principles band gap criterion is applicable to different series of energetic crystals with similar structure or with similar thermal decomposition mechanism. This criterion may be useful for molecular design of high-energy density materials.