What it is: Several different conditions that appear to be caused by an abnormal immune response that dries out the skin, making it itchy and prone to rashes. The most common kind of eczema is atopic dermatitis, which can be set off by stress, sweating, allergens (dust mites, certain foods) and which is linked to asthma and hay fever. Another kind is contact dermatitis, caused by an allergy to a substance, such as perfume or plants. Seborrheic dermatitis occurs when dead skin cells exfoliate abnormally.

Symptoms: Itchy, irritated skin on the insides of your elbows, backs of the knees, or on your hands and face. If you have a very flaky scalp, seborrheic dermatitis is likely to blame. It typically causes a greasy buildup of skin cells on your scalp and the T-zone on your face, especially around your eyebrows.

How to treat it: For atopic and contact dermatitis, use a fragrance-free moisturizer frequently, limit bathing and exposure to irritants. For seborrheic dermatitis wash with a gentle cleanser to remove oil and use a medicated shampoo. Cold compresses are sometimes prescribed to relieve itching. For severe cases, doctors typically prescribe a topical or oral steroid. A new class of drugs called topical immunomodulators is now also being used to reduce the immune response associated with eczema.

Product recommendations: Aveeno Soothing Bath Treatment, $7, with finely ground oatmeal, relieves itchiness. Osmotics TriCeram, $30, contains epidermal lipids to reduce dryness and irritation.

What it is: Hormonal fluctuations boost the sebaceous glands' oil production, increasing the likelihood that pores get plugged with oil, dead cells and bacteria. Stress, too, may influence the release of hormones that affect the skin.

Symptoms: Whiteheads, blackheads, cysts or pimples—plugged pores—on your face, neck, chest, back, shoulders or upper arms.

How to treat it: Keep your hands off your face and wash gently—squeezing and vigorous scrubbing can exacerbate acne. Consult a dermatologist. "Teen acne responds to drying gels, but adult skin needs a gentler remedy," says Polisky, who advises older patients to use Rezamid, a topical cream and cover-up that's less drying than benzoyl peroxide. Certain oral contraceptives have been proven to help control acne, and a new drug, Nicomide, is promising.

Product recommendations: Aveeno Clear Complexion Foaming Cleanser, $7, and L'Oréal Pure Zone Skin Relief Oil-Free moisturizer, $8, contain salicylic acid, a gentle beta-hydroxy acid that helps prevent breakouts.

What it is: A condition activated by anything that triggers the body's defenses (such as a cut, burn or infection like strep throat), causing skin cells to run amok. Responding to a signal to heal, the body produces new skin cells more rapidly and in greater numbers than can be incorporated into the surface layer. The cells accumulate, creating flaky, itchy patches.

Symptoms: Small red bumps that progress to thick, scaly patches on your scalp, elbows, knees, hands and feet.

How to treat it: Though psoriasis is incurable, about 75 to 80 percent of the cases are mild, and treatment can diminish symptoms. Mild cases are treated topically with hydrocortisone, vitamin D- or vitamin A-derivative creams, and preparations that contain small amounts of coal tar, the oldest remedy for psoriasis. Sunlight also helps, but because of the risk of skin cancer, moderation is key. Severe cases of psoriasis are often treated with ultraviolet light in a doctor's office and/or oral retinoid medications.

Product recommendations: Neutrogena T/Gel Shampoo, $5, contains coal tar. The minerals in Ahava Dead Sea Bath Salts, $9, are a natural resource for combating irritation. Aquaphor Healing Ointment, $6, is fragrance- and preservative-free.

What it is: Abnormal blood flow to the vessels in the face makes the skin red, bumpy and swollen. The cause is unknown, though stress can cause outbreaks.

Symptoms: You blush easily or develop ruddiness that comes and goes on your cheeks and nose. Or you might develop a red spot or bumpy patch on your face and swelling along the folds between your mouth and nose. The appearance of blood vessels may increase as they dilate or break. In the most severe cases, excess tissue grows on the nose.

How to treat it: Avoid known triggers: spicy foods, hot beverages, caffeine, alcohol and heat (sweating can also activate rosacea, so try to stay cool). Because sunlight worsens the condition, wear a full-spectrum sunscreen with UVA and UVB protection. Wind is another exacerbating factor. Doctors generally prescribe the antibiotics metronidazole (topical) or tetracycline (oral), both of which decrease inflammation. A light alpha-hydroxy peel can help. In very visible cases, laser surgery or electrosurgery can be used to cauterize dilated blood vessels.

Product recommendations: B. Kamins Booster Blue Rosacea Treatment, $62, and DDF Rosacea Relief, $47, both contain patented anti-inflammatory ingredients: Bio-Maple and Gatuline A, respectively.