If there's one ingredient lauded more than any other for its wrinkle-fighting, complexion-perfecting abilities, it's this derivative of vitamin A. "Here's the deal with retinol," explains Hirsch. "We were talking about it in 1975, and we're still talking about it now because it works." In study after study, retinol has been shown to build collagen, decrease fine lines, improve skin's texture, and fight acne.

The prescription version (retinoic acid, or Retin-A) acts fastest, but it's pricey—and it can be drying. Over-the-counter retinols take eight to 10 weeks to show results (compared with six weeks with an Rx), but are normally paired with anti-inflammatories to calm the redness, peeling, or dryness; they can also cost less than a prescription, depending on your insurance, generally starting around $100.

Whichever type you use, you'll want to ease into your retinol use slowly. "I start patients on the mildest version, one night a week at the onset," says New York City dermatologist Amy Wechsler. As your skin begins to tolerate a pea-size amount, you can eventually go up to two nights a week. But stay off harsh physical scrubs and peels while you're using retinol; remember to moisturize, moisturize, moisturize; and use extra sunscreen for the first six months.