Ice clouds are an important factor in the Earth’s climate system. The aerosols on which ice crystals form have been studied in situ as well as collected and returned for laboratory analysis. Measurements from multiple locations and multiple seasons indicate that aerosol composition has a significant impact on the nucleation of the ice phase. One driver of the nucleation process is the surface composition of the particles, specifically its similarity to water ice. Laboratory studies are used to understand specific activation conditions required for ice nucleation by these particles. Laboratory and field data are combined to understand cloud formation and atmospheric abundance and lifetime. For example, bare mineral dust and metallic particles are highly enhanced in the ice phase when compared to the same particles with organic and sulfate coatings. Particles such as soot and biological material are not found due to a poor crystalline match or low abundance.