This review paper critically analyzes the importance of monitoring the synthesis of plasmonic and optoelectronic materials to not only provide a mechanistic understanding of their nucleation and growth but also crucial kinetic insights to enable their future development. Light-interacting nanoparticles present strong size-properties 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 or so, the development of in-situ and online characterization capabilities has transformed our understanding, triggering a number of mechanistic models. In addition, time-resolved data have been able to reveal the steps rate, even for phenomena taking place in the microsecond scale (i.e. nucleation) thanks to the use of micro flow reactors. However, the literature contains a few disagreements and inaccuracies, which we consider to be due to the general lack of attention and control on mixing (especially relevant when mixing time is comparable to the reaction time) and the presence of additives during synthesis (e.g. stabilizers). Finally, we believe that recent in-situ monitoring development coupled with careful reactor design brings unique opportunities to not only synthesize nanoparticles in a reproducible and controllable manner but also to use data-rich approaches for self-regulated and automated systems.