The development of new synthetic methods for semiconductor nanocrystals dates back to their discovery in the 1980’s. Since then, modern nanoscience has developed countless methods for the colloidal synthesis of a wide variety of materials with exceptionally narrow size distributions. For semiconductor nanocrystals, the observation of narrow size distributions is commonly attributed to a rapid “burst” of nucleation that precedes a prolonged growth period. In this talk I will describe efforts to control the synthesis of InP nanocrystals through the development of tunable precursors. We find that the reactivity of the molecular starting material dictates the optoelectronic properties of the nanocrystalline product. Using this precursor platform and a previously developed method for synthesizing lead chalcogenides, the mechanism leading to narrow size distributions is investigated. We find that monodisperse ensembles of InP and PbS nanocrystals can be synthesized despite periods of long, continuous nucleation. In the most extreme cases it is found that nucleation persists for >50 percent of the total reaction time while retaining a narrow size distribution—in stark contrast with classical views of semiconductor nanocrystal formation. The observation of elongated nucleation periods across a range of materials and its implications for understanding nanocrystal growth processes will be discussed.