One of the most ubiquitous skin care products may also be among the most misunderstood.


In the skin care spectrum, one often needs to look no further than a product’s name to understand its purpose: moisturizer, eye cream—they are entirely self-explanatory. But serum can be a bit more enigmatic. While most may understand instinctively that a serum should be part of their daily routine, there is such a far-reaching array of options nowadays that the narrowing-down process can feel daunting. With that in mind, we asked top dermatologists and brand pioneers to help navigate the what, how, when, where, and why of serums so they will no longer be a mystery.


Most often serums are more nutrient rich and have a lower viscosity (i.e., more sheer and lighter weight) than their moisturizer counterparts. “They have a smaller molecular composition, so it’s possible to create a more intensive concentration of active ingredients, which can penetrate deeper into the skin,” says Dr. Yannis Alexandrides of 111Skin. Natural skincare guru Tata Harper likens serums to skincare’s big workhorses. The high-horsepower wonders come in many guises—liquid, gel, and even oil. “Your skin is like a Spanish-tile roof with all the cells overlapping each other, but when it’s dry or irritated they pull apart,” explains New York–based dermatologist Dr. Ellen Marmur. “Serums saturate the tiles—the cells—and make them expand and seal together so skin looks plumped up and smoother.”


Think of serum not as a skincare solo act, but an essential supporting player. “It should not be the only step of your routine, but an add-on to a moisturizer. Use it to target specific issues like pigmentation,” says Dr. Timm Golüke, founder of Royal Fern. Because serums are highly concentrated, less is more (“five drops will cover a lot of surface area,” adds Marmur), and they do their best work against a backdrop of slightly damp rather than bone-dry skin. As for application order, consider the texture, advises Harper. “With your skincare products you always want to go from the thinnest to the thickest,” she says. “So things with the lowest molecular weight go first, then you work up. If you start with something that has a big molecule, the thinner won’t penetrate.” A typical regimen might go as follows: liquid-y hyaluronic serum, then moisturizer, then oil serum, whose particles are the biggest. Or you can think about product order in terms of priority, says Marmur. “If you have melasma you might want to use your pigment serum first because that takes priority,” she explains. “Then antioxidants, which intercept pollutants, and sun protection would be the last layer.”


Tailor the serums in your repertoire to your skin concern. They are super potent, so they are best able to get to the heart of the matter. (It also means that a patch test, to ensure that your skin isn’t reactive, is never a bad idea.) If you’re acne prone, look for a serum with vitamin C or the gold standard, retinol—both of which promote cellular turnover and reduce inflammation—or, suggests Marmur, lactic or salicylic acid. For extreme dryness, a serum with niacinamide or hyaluronic acid (like the Dr. Barbara Sturm Hyaluronic Acid) will help skin act as a superior sponge for moisture. An oil-based product (like Vintner’s Daughter Active Botanical Serum) will also hit a real hydrating sweet spot, says Marmur. For issues of dullness, natural brightening agents like algae are a favorite of Harper (it’s in her Concentrated Brightening Serum), or Marmur suggests seeking out ferulic acid, which combats free radicals, spurs cellular repair, and prevents more pigmentation. Finally, for deep wrinkles, both Harper and Alexiades name-check peptides (they’re in 111 Skin’s Y Theorem Repair Serum) for their Botox-like effect on the complexion.

And while we often shift our skincare wardrobe from season to season, Harper suggests sticking to a serum program year-round and swapping out your moisturizer instead. “Just change the hydrating part of the routine by season, going for something heavier in the colder months when you’re around more recirculated dry, heated air,” she adds. Or, in the summer, when you want to feel like you have less on your skin, forgo moisturizer and piggyback two serums instead, suggests Marmur.


While in the serum realm layering is certainly encouraged, there are a few hard-and-fast rules to bear in mind, no matter what your skin proclivities, before playing mixologist. For any complexion that’s historically reactive, Golüke cautions against layering vitamin A or retinol with vitamin C or any fruit acids. “It tends to be too aggressive and can lead to irritation,” he says. If both are on your serum roster, just use them at opposite times of the day instead stacking them on top of one another. Marmur also counsels retinol serum devotees to steer clear of using it in tandem with products based on salicylic or lactic acid to prevent possible irritation. To really play it safe, she likes to seek out one product that is formulated with a few of these hero ingredients. “If you’re nervous about it, rather than buying a number of different bottles of serum I would go for one that has been thoughtfully formulated by chemists to include a number of these active ingredients,” she explains. Most important, don’t take a grin-and-bear-it approach to skin reactions if and when they do arise. “While you should usually give a serum at least four weeks—because of the normal cell-turnover rate of our skin—to see if it’s a good add-on, if you see redness or irritation or experience itchiness, there’s no rule that says you have to continue using it,” says Golüke.