Using an anti-aging serum in your skin-care routine is like eating avocado and microblading: Everyone and their mother is doing it these days. But what exactly is a serum, how do you know if you should be using one, and which one is the best for you? We sought out advice from some of the best dermatologists to break it all down.

First things first: A serum is essentially a lightweight moisturizer with super-potent active ingredients that are small in size. This allows serums to penetrate deeply into the skin so that ingredients are delivered straight to their target, explains New York City dermatologist, Neil Sadick, M.D. Most serums are created to combat free radicals for anti-aging benefits, and to also provide an extra layer of moisturization, he adds.