What It’s Meant to Do: A primer is designed to temporarily smooth skin on the lips, the eyelids, or the face with silicones and polymers. “Most contain waxes that bond with cosmetics to give them longevity,” says Jim Hammer, a cosmetics chemist at Pharmasol Labs, in Easton, Massachusetts.

So Who Needs It? “Someone with fine lines around her mouth can benefit from a lip primer, since it keeps lip color from feathering,” says Jeannette Graf, an assistant professor of dermatology at New York University Medical Center, in New York City. Women who have trouble with shadow that creases might consider an eye-makeup base. Makeup primers are ideal for those with sunken scars, since they level the skin.

RS Pick: Laura Mercier Makeup Primer, $30, www.lauramercier.com.

Who Can Skip It: People who are prone to allergies, contact dermatitis, or milia (tiny cysts that form under the eyes). And a primer won’t do much for those with even, velvety complexions.