Find it in: SkinCeuticals C E Ferulic Combination Antioxidant Treatment and Ole Henriksen Truth Serum Collagen Booster

Plays well with: Glutathione, peptides, and copper. "Glutathione is an antioxidant that acts like a battery charger for other antioxidants, including vitamin C, to make them more powerful," says Doris Day, a dermatologist in New York City. (You can find it in Perricone MD AcylGlutathione.) "And at night, vitamin C helps skin cells turn over, which works synergistically with wrinkle-smoothing peptides or copper." (We like Olay Regenerist Regenerating Serum.)

Does not play well with: Glycolic acid or salicylic acid. Like an old-school diva, vitamin C melts down so easily. And since salicylic and glycolic acids alter the pH of the formulas you mix them with, they'll wipe it out before it has done you any good, says Day.