Typically applied before moisturizers to help lock in moisture.  A serum is able to hydrate more effectively than even the heaviest creams. The molecules are able to truly penetrate the skin and hydrate on the deepest level, while the heavier creams more so sit on those top layers. For this reason, you should always apply a serum directly to your skin after cleansing or toning and before your moisturizer  and sunscreen. ... They're not meant to replace your moisturizer , but can be used to boost its hydrating effects. Serums are used to address a variety of skincare issues including but not limited to eczema, psoriasis and hyper-pigmentation.