Quenching is a process of cooling a metal at a rapid rate. This is most often done to produce a martensite transformation. In ferrous alloys, this will often product a harder metal, while non-ferrous alloys will usually become softer than normal. Quenching is a very important part of the induction process in order to get the desired hardness of a metal, and improper quenching may lead to a variety of problems. Various type of aqueous quenchants for induction hardening will be reviewed along with the three stages of the quenching process. Basic information on care and maintenance of those quenchants will also be reviewed.