Acne is a very common dermatosis with characteristic clinical features. It is a polymorphic disease. The clinical expression ranges from non-inflammatory closed and open comedones to inflammatory papules, pustules, and nodules. Most patients have a mixture of non-inflammatory and inflammatory lesions, although some have predominantly one or the other. Acne varies in severity from a very distressing, socially disabling disorder to a state that has been regarded as physiological by some authors. The most severe forms of acne are acne fulminans and acne inversa. Although acne may occur in all age groups, it is most prevalent during adolescence. It is not known why acne subsides in most patients but persists into adulthood in some. Certain medications may be associated with provocation, perpetuation, or exacerbation of pre-existing acne or with acneiform eruptions. Acne-like disorders include rosacea, pseudofolliculitis barbae, and other conditions that share clinical features with acne.