Rapid precipitation is an efficient approach for engineering materials at the nanoscale. By controlling solute nucleation and growth rates in solvent precipitation processes, the size of the resulting nanoparticles (NP) can be controlled. This review discusses Flash Nanoprecipitation (FNP), a technique developed for NP formation using copolymer stabilization. FNP has been explored for various applications, including NP formulation for drugs and imaging agents. The review highlights mixing considerations, supersaturation requirements, and stabilizer selection to provide controlled size NP via FNP, and includes a summary of current understanding of the FNP process, as well as relevant examples and applications.