Alpha hydroxy acids and beta hydroxy acids are the workhorses of your skin care routine. They exfoliate dull skin and fade imperfections, minus scratchy microbeads. Pretty much everyone can benefit from having acid in her routines -- but you shouldn't pick just any ol' AHA cleanser. If you wanna get nerdy about it, the various alpha hydroxy acids and beta hydroxy acids (like salicylic acid, glycolic acid, lactic acid and malic acid) work differently on a molecular level, and choosing which one to use can make or break your skin. We asked Dr. Elizabeth Tanzi, M.D., a Washington, D.C.-based dermatologist, to decode some of the most common AHAs and BHAs and tell which one you should be using.