It is useful to compare and contrast the functional groups and their orientation when Vilsmeier reagents induce either cyclization to form a benzene ring or aromatization of an existing ring. The aromatization of anthrone by Vilsmeier reagents to give 10-chloro-9- anthracenecarboxaldehyde has been long known, and proceeds via the cation 1-ketotetrahydrocarbazoles which imparts an intense deep red colour to the reaction mixture. The effects of temperature and stoichiometry of Vilsmeier reagents can be crucial to the outcome to the reaction of tetrahydrocarbazoles. The chapter presents rings formation by Heteroaromatization. 9-Benzyl-1,2,3,4-tetrahydrocarbazole is formylated in the 1-position at low temperatures in 90% yield. 5-Pyrazolones usually react with Vilsmeier reagents to give formylated pyrazoles. Introduction of chlorine into the 5-oxopyrazolone system 36, in addition to an effectively diformylated 3-methyl group, has been observed using Vilsmeier reagents.