Nucleation in atomic, molecular, and colloidal systems. Nucleation is the first step in the formation of many materials. The simple picture of nucleation, inherent in classical nucleation theory, that has prevailed for more than a century does not account for complex multistep nucleation pathways observed in recent years in experiments and simulations. This issue illustrates and describes the many complex nucleation pathways seen across a range of material systems. The cover shows an artistic rendition of nucleation from a supersaturated calcium carbonate solution. The structures of the initial clusters that form as predicted by molecular dynamics simulations are depicted in full atomistic detail in the foreground and in red in the background. The simulations further predict that the clusters are dense liquid droplets that must later dehydrate to form the solid phase. Results like these call into question the century-old view of how crystals nucleate from solution. Artwork by Adam Wallace. See the technical theme that begins on page 357.