Ice crystals in supercooled clouds may form upon ice nuclei, or they may arise through secondary processes. Two of these secondary ice “multiplication” mechanisms are discussed in some detail: the rime-splintering process and the mechanical fracture of ice particles. The nature of the water-drop size distribution has an important bearing on secondary ice production. Confident predictions of ice particle concentration can only be made in a few limited cloud situations. This is a serious handicap in assessing the feasibility of artificial rainmaking through the ice crystal process.