Contact dermatitis is an inflammatory skin disease due to contact with different substances and caused by two mechanisms: Allergy and irritation. Contact dermatitis is a common skin disease and the most common occupational dermatosis. Differentiating between allergic and irritant contact dermatitis is essential in the identification of etiologic factor(s) and hence, its control and prognosis. This differentiation can be done only by patch test. Unfortunately, this simple, safe and useful test is always neglected. We review the basics of patch test, the method of performance, reading and interpretation of its results, possible side effects and results of studies done with patch test in different countries.