Acne scarring is effectively caused by the body putting too much effort into healing the wounds caused by acne, resulting in an excess of collagen build up in the dermis. This collagen can stay in place permanently, creating acne scarring.
In some cases, the result is not a true scar, but simply an excess of pigmentation. This may fade away over a few months or years – it generally isn’t permanent as with other acne scars.