Methylisothiazolinone (MI) is a preservative commonly used in water-based personal care products. Increases in the allowable concentration of MI alone in these products has led to an epidemic of allergic contact dermatitis (ACD). Although personal care products are the most common source of MI contact allergy, other novel exposures include household products, industrial chemicals, paint, slime, and adhesive agents. Other isothiazolinones such as benzisothiazoline (BIT) and octylisothiazolinone (OIT) are uncommon in personal care products but have been found in leather products, glue, industrial chemicals, paints, and cleaning products. There may be cross-reactivity between OIT and MI, and a minority of patients who are allergic to MI are cosensitized to BIT. In this article, we review MI and related isothiazolinone chemicals.