All products featured on Allure are independently selected by our editors. However, when you buy something through our retail links, we may earn an affiliate commission.When your scalp is really oily, your hair can get weighed down and styling can become more difficult than it ought to be. If you have greasy hair that’s increasingly hard to maintain, you’ll be delighted to know that it’s actually not all bad. According to experts, that oil serves a purpose that’s really good for you.”We all produce an oily substance called sebum on our skin through our oil glands, otherwise known as our sebaceous glands,” says New York City board-certified dermatologist Charlotte Birnbaum. “Sebum improves the barrier of our skin and scalp, and thus, protects our skin and scalp from damage and drying out.”Nevertheless, when it comes to sebum, you can have too much of a good thing, especially if you have fine or straight hair that doesn’t require as much moisture as thick or curly hair. There are a few key reasons why someone might have a perpetually oily scalp, one of which is shampooing too often or using a shampoo that’s too clarifying (maybe even a combination of the two).”Harsh shampoos strip the hair and scalp of natural oils, particularly if they have a basic pH,” explains board-certified dermatologist Mona Gohara, who is based in Hamden, Connecticut. “They open what should be a closed hair cuticle, [then] water evaporates out and dryness follows. The oil glands don’t like that this is happening and kick into overdrive.”As she points out, your scalp is a lot like the skin on your face. “Just like I tell my patients who have an oily face, over-washing and over-scrubbing can just stoke the fire,” she says.If it sounds like that might be the case for you, thankfully, you can help your scalp lessen its own oil production. Below, experts break down why your hair and scalp are greasy, how you can maintain it, and when you can expect to see results.Why is my hair so greasy?As Gohara already explained, over-shampooing is a big cause of constantly greasy hair, but how can you tell if it’s the cause of your greasy hair? According to the experts, that all comes down to how long your hair and scalp have been prone to oil. Gohara says that if your oil-prone scalp is a relatively new development, your hair habits might be the cause. In that case, you can change up your routine to see if it yields any results (more on how you can do that in a minute).