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The formation of crystalline solids from solution is fundamental to many natural and industrial processes. Crystallization may even be the key to the formation of life itself . The crystallization process begins with nucleation, which plays a central role in determining the structure and size distribution of the crystals. In the past decade, experimental and molecular modeling studies of ionic materials such as calcium carbonate, proteins, and organic molecular crystals has suggested that nucleation of solids from solution does not proceed via the classical pathway but follows much more complex routes. These routes are generally referred to as two-step nucleation, but actually encompass a number of potential mechanisms (–). On page 885 of this issue, Wallace et al. use molecular simulations to probe CaCO₃ nucleation. The results provide evidence for a dense liquid-liquid phase in which solvated CaCO₃ clusters come …