In obtaining uniform products, it is necessary to decouple growth from nucleation during a colloidal synthesis of nanocrystals. We recently demonstrated that nucleation and growth could be separated from each other by re-designing the experimental setup while manipulating the reduction pathway of the precursor. Specifically, we pumped the reaction solution through a tubular flow reactor and trigger a single burst of nucleation by subjecting the solution to an elevated temperature for a very short period of time to enable solution-phase reduction. The solution was then kept at room temperature for slow growth through surface reduction, producing pure and uniform nanocrystals. Due to the elimination of additional nucleation events during growth, the products exhibited high uniformity in terms of both size and shape. We also elucidated the mechanistic details by quantitatively analyzing the reduction kinetics and monitoring the nanocrystals obtained at different stages of a synthesis. We further investigated the impacts of both temperature and duration of nucleation on the diversity of seeds and the quality of resultant nanocrystals. This methodology has been extended to the preparation of both Pd and Pt nanocubes and is potentially applicable to the synthesis of other types of colloidal nanocrystals.