Dandruff
Dandruff (also called scurf and historically termed Pityriasis capitis) is due to the excessive shedding of dead skin cells from the scalp.  As it is normal for skin cells to die and flake off, a small amount of flaking is normal and in fact quite common.  Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation. Most cases of dandruff can be easily treated with specialized shampoos. Dandruff is not an organism like lice; it is just dead skin that accumulates in the scalp. Dandruff is unlikely to be the cause of hair loss.
Excessive flaking can also be a symptom of seborrhoeic dermatitis, psoriasis, fungal infection or excoriation associated with infestation of head lice.
Dandruff is a global phenomenon and many people find that dandruff can cause social or self-esteem problems. Treatment may be important purely for psychological reasons.

# Causes
As the epidermal layer continually replaces itself, cells are pushed outward where they eventually die and flake off.  In most people, these flakes of skin are too small to be visible.  However, certain conditions cause cell turnover to be unusually rapid, especially in the scalp. For people with dandruff, skin cells may mature and be shed in 2 - 7 days, as opposed to around a month in people without dandruff.  The result is that dead skin cells are shed in large, oily clumps, which appear as white or grayish patches on the scalp skin and clothes.
Dandruff has been shown to be the result of three required factors:
- Skin oil commonly referred to as sebum or sebaceous secretions
- The metabolic by-products of skin micro-organisms (most specifically Malassezia yeasts)
- Individual susceptibility
Common older literature cites the fungus Malassezia furfur (previously known as Pityrosporum ovale) as the cause of dandruff.  While this fungus is found naturally on the skin surface of both healthy people and those with dandruff, it has recently been shown that a scalp specific fungus,  Malassezia globosa, is the responsible agent.  This fungus metabolizes triglycerides present in sebum by the expression of lipase, resulting in a lipid byproduct oleic acid (OA). Penetration by OA of the top layer of the epidermis, the stratum corneum, results in an inflammatory response in susceptible persons which disturbs homeostasis and results in erratic cleavage of stratum corneum cells.
Rarely, dandruff can be a manifestation of an allergic reaction to chemicals in hair gels/sprays, hair oils, or sometimes even dandruff medications like ketoconazole.
There is no convincing evidence that food (such as sugar or yeast), excessive perspiration, or climate have any role in the pathogenesis of dandruff.

# Seborrheic dermatitis
Flaking is a symptom of seborrheic dermatitis. Joseph Bark notes that "Redness and itching is actually seborrheic dermatitis, and it frequently occurs around the folds of the nose and the eyebrow areas, not just the scalp."  Dry, thick, well-defined lesions consisting of large, silvery scales may be traced to the less common psoriasis of the scalp.
Seasonal changes, stress, and immuno-suppression seem to affect seborrheic dermatitis.

# Treatment
There have been many strategies for the control of dandruff.  Simply increasing shampooing will remove flakes. However, elimination of the fungus results in dramatic improvement. Regular shampooing with an anti-fungal product can reduce recurrence.
Anti-fungal/anti-dandruff shampoos containing ketoconazole have been shown to be more effective than zinc pyrithione. Ketoconazole is the most effective antifungal agent concluded by multiple studies.