By now you know the drill: Every few months a new wunderkind skin care ingredient is discovered in some remote locale, and pretty soon it’s everywhere—in your masks, serums, foot creams, insert-step-in-your-beauty-routine-here. But at the end of the day, there are only a handful of ingredients that have stood the test of time and truly become essential. “In skin care, they’re the holy grail,” says Cambridge, Massachusetts, dermatologist Ranella Hirsch.

You’ve probably heard of all these by now. (Retinol, hyaluronic acid, AHAs, peptides, and vitamin C all make the list.) But you may still be a little confused on what exactly each one does—and how you should be using them. Here, we break it all down.

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