Aromatic and heterocyclic amines are a major class of chemical mutagens and carcinogens. The toxicity of these compounds is a consequence of their metabolic activation. The best-characterized enzymatic pathways for aromatic amine activation lead to the formation of reactive esters such as N-acetoxyarylamines, which are believed to be precursors of short-lived nitrenium ions. In the 1960s, nitrenium ions were invoked as likely intermediates in the formation of arylamine-derived DNA adducts. More recently, nitrenium ion chemistry has been studied by methods such as trapping with azide ion, laser flash photolysis, and the preparation of highly stabilized of examples (e.g., dianisylnitrenium ion). In this review, we discuss the development of our understanding of nitrenium ion chemistry, with emphasis on their generation in biological systems and their reactions with critical targets such as DNA.