A fundamental understanding of precipitation reactions is based on information about the structural evolution of the system on all length scales and starting at the beginning of the reaction. We introduce X-ray microscopy as an additional analytical tool in order to achieve this understanding. X-ray microscopy combines high resolution (down to 40 mo) and experiments at ambient pressure. The latter point allows the study of colloidal systems within the water phase and the performance of time-resolved microscopy. Taking CaCO3 precipitation as a model system, it is shown how further knowledge can be obtained on the dynamics of structure formation. We discuss the following points: formation and recrystallization of precursors; transient stabilization of CaCO3 nanoparticles by polycarboxylates; dependence of the structure evolution (stabilization or recrystallization) on the polymer concentration.