Skin care technology is constantly changing, and we're loving the new crop of do-it-all serums on the market. But what makes a serum different from a moisturizer? These gel-like formulas have a more concentrated amount of active ingredients in the mix, targeted to battle slick skin, signs of aging, and everything in between. Since the molecules are small, they penetrate into deeper layers of the skin better than a moisturizer, making them perfect for complexions that need a lasting pick-me-up. A little goes a long way, so layer on a dime-sized amount immediately after washing your face, but be sure to read the directions carefully. While some serums can be worn during the day, specific types should only be applied before going to bed. Click through the gallery to see some of our favorites.