Few things in beauty are as controversial as exfoliation. Beyoncé's dermatologist recommends exfoliating once a day. Other dermatologists say two or three times a week. Still, others recommend once a week.
So what's the real answer? Are the people recommending frequent exfoliation right, or are they just trying to sell you more skincare products?
Let's investigate.
The Benefits of Exfoliation
First of all, we can answer one question right off the bat. Should you exfoliate your skin, or is it all just snake oil hoping to sell you products you don't need?
Exfoliation is a powerful and beneficial skincare technique with a lot of potential benefits. You're not going to look like a troll if you don't do it, but there is a clear difference between doing it and not doing it. The only real risk is doing it too often, which we'll get into later.
So what are the various benefits you get from exfoliating? Here's a rundown.
Exfoliation helps unclog your pores. Acne, whiteheads, and other skin issues are largely caused by a buildup of crud in your pores. Dirt, oil, and dead skin cells get stuck under dry skin in the pore and gets infected. It's not a dangerous infection – your body knows how to fight it off easily – but it does cause a bit of localized swelling in the form of a pimple. When you exfoliate, you remove both the oils and the dead skin that seal the pores and thus minimize or prevent breakouts.
It enhances other skincare products. A lot of beauty products work best when they can penetrate deeply into your skin. The more dead skin, oil, and crud there is built upon your skin, the harder those skincare products have to work to have an effect. You end up needing more of them, or having patchy coverage, or just having low-efficacy products in your routine. Exfoliation helps open up the skin and makes it more "vulnerable" to these beneficial products, so they can penetrate deeper and have a stronger effect.
It can help remove blemishes. There are a wide range of different things that can cause a blemish on your skin. A scar, be it from a wound or from acne, leaves a discolored blemish behind. Hyperpigmentation, dark spots, rough textures; all manner of skin issues might not be dangerous at all, but can be disheartening, a blow to the self-esteem. Exfoliation, through scraping away the outer layers of skin, helps cycle through blemishes and restore the healthy skin beneath. They won't go away immediately, but through months of dedicated exfoliation, you can even remove scarring.
It can stimulate the growth of new skin. The primary mechanism of exfoliation is scrubbing away the outer, dead layers of skin to reveal the fresh, vibrant, healthy new skin beneath. Skin is your barrier against the outside world, and your body wants to keep it nice and thick, so when you expose new skin, your body starts further producing skin beneath it to keep that layer nice and protective. Combine this with something that fuels skin growth like collagen, and you have a recipe for thick, protective, healthy skin.
It helps boost blood flow. Another side effect of stripping the outer layers of skin is that it brings blood closer to the surface, even if it's only by a few micrometers. The presence of blood closer to the surface is what's responsible for that "healthy glow" we so often seek. It doesn't last indefinitely, which is why you need to exfoliate regularly, but it's great for the few hours or the few days after you exfoliate.
You might see a few other benefits listed on other sites talking about this same subject, but for the most part, they're covered up above. For example, it helps reduce acne flare-ups, through unclogging pores. We didn't feel we needed to mention that twice, so we left it out.
Types of Exfoliation
There are two kinds of exfoliation, and they work in different ways. These two options are called mechanical exfoliation and chemical exfoliation.
You can think of it the same way you might think of cleaning the crud on a pan or dish. You can take a sponge, brush, or steel wool and scrub the stubborn crud; that's mechanical exfoliation. Mechanical exfoliation uses some kind of physical abrasive, whether it's a rough pad or brush, or microbeads in cream, to physically scrape away the outer surface layers of skin. 
Scrubs, as mentioned in the title of this post, are usually mechanical exfoliation products. Sugar or coffee scrubs, for example, use the graininess of the sugar or coffee to take the place of microbeads to scrape away at the outer layers of skin. Professional products that use microbeads are also a kind of scrub.
On the other hand, you can soak the crud in soap and let the soap dissolve away the bonds holding it in place, then wipe it away or just rinse it clean. This is a chemical exfoliation. You use a chemical, typically a light acid, to dissolve away the oil, the bonds between dead skin cells, and the crud on the surface of your skin. Then you rinse it all away.
There are also combinations of the two; using a light acid to dissolve those bonds, and then using a pad to scrub it all away. This is generally a deep exfoliating treatment and might not be safe for everyone, but it can be useful against particularly stubborn kinds of blemishes or skin damage.
What Happens if You Over-Exfoliate
The way exfoliation works is basically just through repeated damage and healing. You're damaging your skin by stripping off the outer layer of dead skin, which is helping to protect the lower, alive skin by being a physical barrier. Things like dirt, grime, disease, and sunlight hit the dead skin, oil, and dirt that's already there, and get stuck.
When you remove this outer layer of protection, you expose the underlayer to further damage. Sunlight can hit that healthy skin and hurt it more. Dirt can clog pores deeper. Collagen breaks down and your body has to work harder to replace it in the scaffolding beneath the outer layers.
If you exfoliate too much, you expose yourself to excessive damage from the environment. Normally, the worst that happens is that you might be a little more vulnerable to sunburn, or you might be at slightly more risk of something like poison ivy damaging your skin when you touch it, or an allergic reaction if you have topical allergies.
This is why most of the time, exfoliation is part of an overall skin care regimen. You're not just exfoliating, you're exfoliating and then using a moisturizer with SPF sunblock capabilities, you're using a protective mask, you're restoring your skin through other means. The people who recommend exfoliation never recommend it alone and neither do we.
If you exfoliate too often, though, what might happen? Well, the biggest potential problem is just wearing out your skin's ability to heal. Think of a pimple or a papercut; it's a small wound, and when left alone, it will heal and disappear to an extent such that you'd never know it was there. If you pick at it, keep opening it up, tearing off the scab, and otherwise continually re-wound it, eventually, your body will throw its metaphorical hands up and take drastic action. Rather than keep with the slow and ineffective wound healing, it will rapidly seal up the wound. That rapid seal is a scar, made up of scar tissue rather than skin layers, and turns into a blemish.
In extreme cases, over-exfoliation can lead to the development of low-level scarring for exactly this reason. You're essentially running your skin under a mild abrasive scrub or an acid constantly, and that damages it. Normally, healing works fine to restore that skin and keep it healthy, but if you're hitting it too hard too often, it will take more drastic action to solve the problem.
You also have to consider that over time, your skin is going to lose elasticity. You can't exfoliate away wrinkles; you can't damage your skin to solve another kind of damage. It's possible that, if you exfoliate too often, you could develop wrinkles a little sooner than you otherwise would.
How Often Should You Exfoliate?
So at the end of the day, how often should you actually exfoliate? Can you hit your skin with a skincare product once a day or more than once a day, or should you go for less than once a day?
As with many such questions, the answer is "it depends." So what does it depend on? Two things.
First, it depends on your skin type. People with normal skin might be able to exfoliate daily, but will be better off if they limit themselves to once every other day. People with dry skin should exfoliate less often since dry skin can be more prone to damage. People with oily skin should exfoliate more often, to help keep their pores clean and keep the oily sheen off their skin.
Most importantly, people with sensitive skin need to be very careful with skincare products, particularly those that can damage the skin, like exfoliating products. Sensitive skin turns red and can be damaged by even light treatments, including soaps, and something as mild as washing that skin and leaving it open to the air can cause splotchy red patches and irritation. These people need to limit exfoliation to only occasional use, use mild products, or skip it altogether.
Secondly, it depends on the kind of product you're using. Both mechanical and chemical exfoliating products come in a wide variety of different strengths. The stronger the product, the harsher it is on the skin, and the more precautions you need to take to keep from really damaging your skin. Chemical exfoliants are generally lighter on the skin, but it's easy to let them go overboard. Letting them sit too long, using too strong a concentration, or using them too often can lead to damage. Mechanical exfoliants are harder to abuse but harsher as a baseline. Scrubs run a wide range of milk to harsh, and should probably be used a little less often than the baseline.
Tips for Exfoliating Properly
To a certain extent, you can adjust how often you exfoliate by adjusting the products you use.
If you want to exfoliate every day, choose every a very light mechanical exfoliating product, or a low concentration chemical exfoliant. Think of it as a sliding scale; the more often you want to use it, the lighter it should be. Daily exfoliation is as frequent as you want to go, and you should use plenty of surrounding treatments as well. 
Specifically, we recommend using a mild soap to cleanse your skin, then use a light exfoliant to exfoliate. Once your skin has been stripped bare, use protective lotions and creams to restore moisture and protect yourself from outside damage. Typically, you'll want to do this in the morning. If you open your skin up to damage in the evening and then sleep with your face pressed to a pillow, you're likely going to break out. This is, though, why it's especially important to use a cream or treatment that has sun protection included.
You can also exfoliate every day if you're a person with oily skin or if you have a specific issue, like acne, that you want to take care of. Again, opting for relatively mild exfoliants is the way to go here.
More likely, for most people with normal skin, you're going to want to exfoliate every 2-3 days. Think of it like weight training; you don't want to push yourself every day, or you risk injuring yourself and doing more harm than good. With exfoliation, think of it like a one-on, one-off kind of pattern, or one-on, two-off. You strip your skin bare, treat it with deep penetrating moisturizers and vitamins, then you give it a day or two to rest and heal from that treatment.
If you have very sensitive skin, try not to exfoliate more than once a week, or skip it entirely if it does more harm than good.
