Cadmium selenide nanocrystals are reproducibly synthesized at high-temperature in continuous flow, chip-based microfluidic reactors and exhibit size distributions comparable to those for conventional macroscale syntheses. Nanocrystal size, probed by fluorescence, is precisely controlled by independently varying the temperature, flow rate, and concentration of precursor solution flowing through heated microchannels. These experiments demonstrate the ability to fine-tune physical properties and test wide ranges of conditions precisely and rapidly inside chip-based microreactors, enabling rational, cost-effective, and environmentally friendly development and production of novel nanostructures.