What Are Those Ingredients in Your Skin-Care Products?

Also known as: Ascorbic acid, L-ascorbic acid, ascorbate, magnesium ascorbyl phosphate, ascorbyl palmitate.

Good for: All skin types, except very sensitive skin.

What it treats: Sun damage, aging.

How it works: Vitamin C neutralizes free radicals, which form when the skin is exposed to sun, pollution, cigarette smoke, and other harmful elements. Free radicals damage collagen deep within the skin, causing wrinkles, blotches, and inflammation.

RS picks: Dr. Brandt "C" Gel, $90, dermstore.com. Cellex-C High Potency Serum, $90, drugstore.com.

Also known as: Beta hydroxy acid (BHA).

Good for: Normal skin, oily skin.

What it treats: Acne, irritation.

How it works: Salicylic acid can penetrate the sebum that clogs pores, clearing blackheads. "When used on a regular basis, it can decrease the frequency and severity of acne eruptions," says Leslie Baumann, a professor of dermatology at the University of Miami, in Florida. Avoid it if you have dry or sensitive skin or an allergy to aspirin. (It's in the same family as the pain reliever.)

RS pick: Neutrogena Rapid Clear Acne Defense Face Lotion, $8.50 at drugstores.

Also known as: 4-hydroxy-3-methoxycinnamic acid.
 
Good for: Dry skin, normal skin.
 
What it treats: Sun damage, aging.
 
How it works: Usually combined with vitamins C and E, "ferulic acid helps stabilize them," says Barbara R. Reed, a clinical professor of dermatology at the University of Colorado Health Science Center, in Denver. This doubles their power as sun-protective agents and boosts their ability to neutralize free radicals.
 
RS pick: SkinCeuticals CE Ferulic serum, $135, skinceuticals.com for store locations.

Also known as: Ubiquinone, coenzyme Q-10.

Good for: All skin types.

What it treats: Sun damage, irritation, aging.

How it works: Idebenone is one of the strongest antioxidants contained in cosmetics. Its anti-inflammatory properties "help preserve collagen and elastin," which give the skin support and resilience, says Lisa Donofrio, an associate clinical professor of dermatology at the Yale University School of Medicine.

RS picks: Priori Radical Defense, $120, prioriskincare.com for stores. Elizabeth Arden Prevage Anti-Aging Treatment, $155, elizabetharden.com.

Also known as: Sodium hyaluronate.

Good for: All skin types, especially dry skin.

What it treats: Aging.

How it works: Hyaluronic acid attracts and seals water into the skin. "When you hydrate the skin from outside, you make fine lines less noticeable," says Susan Weinkle, an assistant clinical professor of dermatology at the University of South Florida, in Tampa. Avoid hyaluronic acid if you live in a low-humidity climate. It will sit unabsorbed on the surface of your skin and may actually dehydrate it, drawing out moisture.

RS pick: L'Oréal Skin Genesis Daily Moisturizer Lotion, $25 at drugstores.

Also known as: Epigallocatechin gallate (EGCG).

Good for: All skin types.

What it treats: Sun damage, irritation, aging.

How it works: "Green tea is one of the most potent forms of photo-protection," says Patricia Farris, a clinical assistant professor of dermatology at Tulane University, in New Orleans. "It protects against sun damage, and studies have shown that a product containing at least 5 percent green-tea extract may help prevent skin cancer" when applied before sun exposure along with sunscreen.

RS pick: Kinerase Gentle Daily Cleanser, $33, kinerase.com.

Also known as: Nicotinamide.

Good for: Dry skin.

What it treats: Sun damage, irritation, aging.

How it works: "Niacinamide stimulates microcirculation and prevents water loss in the skin," says Reed. Additionally, says Donofrio, "deep in the skin, it increases production of ceramides, which are the glue that holds the cells together and prevents water loss." So it helps to hydrate, too. Translation: It gives skin a healthy, youthful glow.

RS picks: Olay Regenerist Micro Sculpting Cream, $27 at drugstores. Luzern Serum Rosacea Care Absolut, $75, luzernlabs.com.

Also known as: Lipoic acid.

Good for: All skin types, except very sensitive skin.

What it treats: Sun damage, aging.

How it works: Alpha lipoic acid "helps with exfoliation and the stimulation of collagen, which may lead to a smoother appearance of the skin," says Ronald R. Brancaccio, a clinical professor of dermatology at the New York University School of Medicine, in New York City. Since it's a powerful exfoliator, avoid it if your skin is sensitive or prone to redness.

RS picks: N.V. Perricone M.D. Advanced Eye Area Therapy, $95, perriconemd.com. Aubrey Organics Rosa Mosqueta Night Creme, $23.50, aubrey-organics.com.