It stimulates new skin growth, promoting collagen and elastin formation. It must be used very, very, very sparingly for best results. Using too much irritates the skin and causes redness. Long term use is almost like a mild chemical peel. I used Retin A many years ago for acne and noticed what great things it did for the general texture of my skin.
Collagen injections can do wonders for small depressions in the skin.
They don't work for everyone though. For me that filled in a scar on my right cheek that I had since birth. Collagen also filled in acne marks that I had. The results have lasted for me. If you buy only one book, buy this one.