Growth of Colloidal Nanocrystals by Liquid-Like Coalescence**

We here describe, model, and predict the growth kinetics of amine-capped PbS colloidal nanoparticles in the absence of supersaturation. The particles grow by coalescence rather than by Ostwald ripening. A comparison of different models indicates that the effective activation energy of coalescence (67.65 kJ mol−1) is associated with two terms: a term proportional to the contact area between the ligand shells of two colliding particles, and a constant term. Our Brownian dynamics simulations show (i) how the remarkably low activation energy (or large rate constants) are most likely due to the large difference in size between the particles and their mean free path of diffusion, and (ii) how the low polydispersity is the likely result of the suppression of collision rates between rare populations due to crowding. The model successfully predicts the growth kinetics of nanoparticles, therefore enabling the precise control of the average particle size without the need of supersaturation.