To comply with certification rules, aircraft must demonstrate their operability in ice crystal icing conditions. This chapter is devoted to ice crystal icing and highlights its specifics compared to classical supercooled water icing. Dedicated models are proposed for the transport of particles, their impact on walls, and the accretion in crystal conditions. Their integration in 2-D and 3-D icing suites is presented. Preliminary validations of the models are presented for both glaciated and mixed-phase conditions as well as a first industrial application.