Oh, what a great question! Thank you for asking! Serums are an amazing addition to any skin care routine. The reason for this being that, skin care companies are able to pack a lot more active ingredients into a serum form, than what they can into a cream or lotion form. Creams and lotions are difficult to pack full of actives because it is then difficult to stabilise the products. So where a cream may contain about between 10-30% actives, a serum can have anything from 30-90% actives (depending on what the active ingredients are and whether it is a single active or a combination of actives, and of course depending on the quality of the product and the manufacturer). Bottom line is, if you want to get the max number of active ingredients on to the skin as possible, applying a serum under your moisturiser will boost your skin uneblievably!