jats:titleAbstract</jats:title>jats:pThis review paper analyzes the importance of monitoring the synthesis of plasmonic and optoelectronic materials to provide a mechanistic understanding of their nucleatiçon and growth, as well as crucial kinetic insights to enable their future development. Light‐interacting nanoparticles present strong size–property relationships, such that size control is at the core of any synthetic development. However, conventional ex situ characterization of these materials has heavily limited their development to simple trial‐and‐error approaches. Over the last decade, the development of in situ and online characterization capabilities has transformed the understanding of mechanistic models. In addition, time‐resolved data are able to reveal the step rate, even for phenomena taking place in the microsecond timescale (i.e., nucleation), thanks to the use of micro‐flow‐reactors. However, the literature contains a few disagreements and inaccuracies, which are considered to be due to the general lack of attention and control on mixing (relevant when mixing time is comparable to the reaction time), and the presence of additives during synthesis (e.g., stabilizers). Finally, it is believed that recent in situ monitoring development coupled with reactor design brings unique opportunities to not only synthesize nanoparticles in a reproducible and controllable manner, but also gives data‐rich approaches for self‐regulated and automated systems.</jats:p>