Dandruff can be annoying, uncomfortable, and embarrassing (and it's often confused with the simpler to treat dry scalp). Dandruff is caused by a fungus called malassezia (which eats fat and lives on the scalp of most healthy adults). Dandruff occurs when this fungus grows out of control and starts feeding on the oil on your scalp, resulting in an increased cell turnover and more dead cells (which combine with dirt and oil to form flakes).
Treatment for dandruff normally begins with an over-the-counter shampoo containing the active ingredient zinc pyrithione (Head & Shoulders), selenium sulfide (Selsun Blue) or coal tar (Neutrogena T/Gel). Use one of these shampoos, as directed, until dandruff is controlled. Should the shampoo fail, try another type. If you can't find a shampoo which works, see your dermatologist who may be able to prescribe a more aggressive treatment.
Scalp psoriasis is a condition of the scalp in which the outer layer of skin cells reproduce faster than normal and the skin develops a build-up of dry, dead skin cells. These cells form thick, silvery scales which can itch, burn, or cause soreness (and, often, may crack and bleed). Unfortunately, scalp psoriasis often requires long-term treatment. Your doctor may prescribe a number of treatments such as topical corticosteroid creams, synthetic vitamin D3 (not the same as over-the-counter supplements), topical retinoid, salicylic acid creams, moisturizers, and coal tar. Other treatments include light therapy and oral medications or injections.
For scalp psoriasis, you should most certainly see a doctor. That said, I have had a few clients who have had great success using a tar-based shampoo such as MG217 Medicated Tar Shampoo. You may also wish to visit the Psoriasis Guide Site on About.com for more information on scalp psoriasis.
Like the acne which appears on the face, scalp acne can be a problematic (yet, less noticeable) problem. Scalp acne is caused by many of the same factors as face acne -- clogged pores due to overactive sebaceous glands or by surface debris. The fact that hair can help trap dirt, oil, and debris, on the scalp adds to the complexity of the problem. To treat scalp acne, shampoo more frequently using a mild shampoo such as Menscience Daily Shampoo which contains salicylic acid -- this ingredient helps shed dead skin and deep cleans the scalp. While you want to stay away from peroxide-based products which may discolor the hair, you may also wish to add a salicylic acid based moisturizer such as Clean & Clear Advantage Oil Free Moisturizer (available at supermarkets and drug stores). Of course, using a moisturizer on the scalp will be much easier if you have short hair. You'll also want to avoid using wax or oil-based styling products (which can build up on the scalp) and I highly recommend washing your hair before bed each night.
Seborrheic Dermatitis is a fairly common skin condition (sometimes called cradle cap) which is very similar to scalp psoriasis and the treatment options are similar. Unlike scalp psoriasis, this condition is normally temporary and can often be treated with the use of an over-the-counter dandruff shampoo or tar shampoo (such as MG217 Medicated Tar Shampoo). Other treatable temporary problems of the scalp include dry scalp and scalp sunburn, which can often be treated by using a basic moisturizing shampoo and conditioner. Often, a healthy-looking head of hair begins with a healthy scalp, so treating scalp problems and caring for your scalp are essential.