Isothiazolinones are used in a wide range of industrial, household, and cosmetic products as preservatives with bacteriostatic and fungistatic properties. However, they are potent contact allergens. The use of methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) in a 3:1 combination (MCI/MI) in cosmetic products led to high rates of contact allergy across the EU in the 1980s. This prompted a restriction of MCI/MI to 15 ppm in the EU, which resulted in a decrease in contact allergy cases. MI was introduced as a standalone preservative in chemical substances around the millennium and in cosmetic products from 2005 onwards. This led to an unprecedented increase in new cases of MI contact allergy globally, with prevalence rates exceeding 7–10% in many countries during 2013–2014. In the EU, the prevalence of MI contact allergy has since decreased to 2–6% following regulatory restrictions on its use in cosmetic products, whereas rates in North America remain above 10%. The two epidemics were mainly driven by the use of MCI/MI and MI in cosmetic products, but both are also important allergens in the work environment. Examples of occupations at increased risk of MCI/MI and MI sensitization are painters, machine operators, mechanics, and healthcare workers. Other isothiazolinones, e.g., benzisothiazolinone (BIT) and octylisothiazolinone, are only used in industrial and non-cosmetic consumer products like paints, detergents, and leather, and pose a problem in special occupations. The prevalence of BIT contact allergy has increased, exceeding 3% in the EU between 2019 and 2022, and reaching 10.4% in North America during 2019–2020.