Whether your skincare routine consists of 15 steps or five, there is one in particular that should always be a priority: serum. This essential offers more targeted results, potent formulations, and customization options. So, what exactly is a serum? “A serum is usually a liquid or gel containing high concentrations of skin actives, which are able to penetrate deeper than other topicals [like moisturizers] with content occlusives that prevent deeper penetration,” explains board-certified dermatologist Dr. Marnie Nussbaum. You’re never too young to start using a serum, Nussbaum shares, and they’re the best way to target specific skin concerns due to their high concentration and potency. It’s important when choosing a serum to know which skin concern you’re hoping to target—most basic serums contain a combination of antioxidants, which will brighten the skin. Looking for something moisturizing? “A hydrating serum should contain hyaluronic acid and peptide,” Nussbaum says. “Hyluaronic acid keeps skin plump and glowing—and basically allows your skin to repair itself easier.” If anti-aging is your main concern, Nussbaum advises looking for a serum that contain retinols (which increase skin cell turnover), glycolic acid, and Vitamin C.