Chapter 12 391 presence of flakes and irritation beyond the scalp.2 It is also clear that dandruff and seborrheic dermatitis are more than just superficial disorders of the stratum corneum. Instead, the epidermis is substantially altered, with hyperproliferation, excess intercellular and intracellular lipids, interdigitation of the corneal envelope, and parakeratosis (Figure 2).12-13 Interestingly, these abnormalities are seen throughout the scalp of affected individuals, not just in areas of flaking. The vast majority of current literature views dandruff and seborrheic dermatitis as a continuum of symptom severity with the same etiology. Furthermore, the concept of dandruff and seborrheic dermatitis as a continuum of symptoms from the same etiology is supported by the presence of inflammatory markers in dandruff even though the inflammation is not visibly apparent.14 Because these two entities share a similar mechanism and treatment for both is similar, we shall refer to these jointly (D/SD) for the remainder of this chapter simply as “dandruff.” Etiology Based upon the most recent evidence, the etiology of dandruff and seborrheic dermatitis appears to be dependent upon three factors: sebaceous gland secretions, microfloral metabolism, and individual susceptibility (Figure 3).15-16 This chapter will describe Figure 2. The physiology of seborrheic dermatitis