Acne vulgaris is an inflammatory disorder of the pilosebaceous (hair follicle) units. It is very common, affecting 90% of teenagers, albeit often mildly. However, its onset may be delayed until the late twenties or early thirties, and very occasionally even later. In the majority of cases, acne settles by the mid-twenties, but for some patients it may still be problematic in their forties and beyond. Patients should be referred to a dermatologist if they have: a very severe variant; severe social or psychological problems; risk of scarring; failed to respond to treatment or are suspected of having an underlying endocrinological cause. [With external links to the evidence base]