A broad spectrum of various substances may cause occupational allergic contact dermatitis. Often, it is difficult to identify potentially or truly relevant allergens at the workplace. Therefore, the consulting physician must have a comprehensive knowledge of possible allergens in different occupations to perform specific diagnostics and to initiate efficient secondary prevention by allergen avoidance. In this review, we give some practically relevant general comments on patch testing in occupational dermatitis, followed by a discussion of important occupational allergens in those occupational groups most frequently affected by occupational dermatitis. Finally, recommendations on which allergens should be tested in the respective occupational groups are given or reported from the literature.