In this work, we studied the heterogeneous nucleation and growth of CaCO3 within regular arrays of picoliter droplets created on patterned self-assembled monolayers (SAMs). The SAMs provide well-defined substrates that offer control over CaCO3 nucleation, and we used these impurity-free droplet arrays to study crystal growth in spatially and chemically controlled, finite-reservoir environments. The results demonstrate a number of remarkable features of precipitation within these confined volumes. CaCO3 crystallization proceeds significantly more slowly in the droplets than in the bulk, allowing the mechanism of crystallization, which progresses via amorphous calcium carbonate, to be easily observed. In addition, the precipitation reaction terminates at an earlier stage than in the bulk solution, revealing intermediate growth forms. Confinement can therefore be used as a straightforward method for studying the mechanisms of crystallization on a substrate without the requirement for specialized analytical techniques. The results are also of significance to biomineralization processes, where crystallization typically occurs in confinement and in association with organic matrices, and it is envisaged that the method is applicable to many crystallizing systems.