Seborrheic dermatitis (SD) is a common skin disorder that mainly affects the scalp, causing scale, itch, and red skin. Dandruff, a subset of SD, shows scale and itch in the absence of associated red skin. Seborrheic dermatitis can occur on many different body areas, including the scalp, eyebrows, eyelids, creases of the nose, lips, behind the ears, in the outer ear, and middle of the chest. On the scalp, SD is commonly associated with pruritus (itch). Multifactorial theories have been proposed regarding the pathogenesis of SD, and these include irritation from a skin-based yeast called malessizia, overproduction of sebum by sebaceous glands in the affected skin areas, and individual variability. Specifically, malassezia produce free fatty acids from sebaceous gland-produced triglycerides, and these free fatty acids have an "irritant" (ie, non-immunologic) effect in those patients with a "susceptible predisposition." Other factors associated with flares of SD include stress, fatigue, weather extremes, infrequent shampoos or skin cleaning, use of lotions that contain alcohol, skin disorders (such as acne), obesity, neurologic conditions (including Parkinson's disease, stroke, and head injury), and HIV infection. What role, if any, such factors play in the etiology of scalp pruritus in patients with SD is not known. Furthermore, itch is signaled by specific subsets of nerves in the skin and whether pruritus of SD results from alteration of these nerves by the underlying inflammatory state is unknown. Moreover, pruritus from the skin can be a result of peripheral sensitization, where skin-derived signals activate nerves in the skin to send increased nerve signals into the central nervous system. Pruritus sensed in the skin can also be a result of altered central-nervous system processing of skin-derived nerve signals, such as when skin-derived touch or pain is misperceived as itch by the central nervous system. Whether itch in patients with SD is a result of peripheral and/or central sensitization is also not unknown. Understanding this distinction has therapeutic implications, in that itch that is a result of misprocessing by the central nervous system will not be adequately treated, in the short term, by skin-administered topical therapies.