The nucleation and growth mechanisms of calcium phosphate were investigated using supersaturated solutions similar to physiological conditions and model self-assembled monolayer substrates tailored to have functional groups that mimicked chemistries found in bone organic matrices. Deposition kinetics were studied using an in situ microbalance and showed a long induction period followed by a second region of extensive growth, both of which depended on the solution supersaturation. Solution studies revealed that the growth of calcium phosphate onto the surfaces after the induction period corresponded to nucleation and growth in solution. Nuclei formed in solution, started to grow, adsorbed onto the substrates, and then grew further to form apatite films composed of coalesced, oriented crystallites. The solution-formed critical nucleus mechanism is in contrast to heterogeneous nucleation and reveals an important mechanism for calcium phosphate growth onto surfaces.