A discussion with refs. In spite of the fact that there is rather little knowledge of the dynamics of nucleation of specific compds., there exists a tool to detect the growing of (larger) crystal nuclei, which can help to get some control on crystn. By Mie-scattering, applied with a com. available particle counter the authors can decide on the appropriate temp. for nucleation and subsequent crystn. Crystal growing can be obsd. by video monitoring. The same app. may be used for testing nucleation inhibitors, which are important for the formation of gas hydrates in pipelines. The in situ crystn. technique, as applied to many compds. which are liq. or gases at ambient, also serves to gain some control on nucleation. For compds. which solidify in a glassy state, nucleation can be initiated by ultrasonic shock waves. Also, mixts. with compds. that work as templates for nuclei can help for such cases. The circumvent of plastic phases can be performed by producing crystal nuclei in the low-temp., ordered phase with subsequent crystal growths.