Most hair serums nourish your hair, hydrate dry hair, and control frizz. "By refining the surface of the hair, they can help to shut the cuticle and create shine as well. A topcoat of serum is a really good way to keep the good stuff in and the bad stuff out while improving elasticity and therefore the manageability of your hair," adds Murphy. "They also can offer protection from environmental factors like pollution, wind, and sun."