Disorders of skin pigmentation may be due to primary abnormalities of melanocyte structure and function, as in vitiligo, or may reflect secondary changes associated with other skin diseases, as one sees in postinflammatory hyperpigmentation and post-inflammatory hypopigmentation in, for example, psoriasis and atopic dermatitis (Chapter 1). The mechanism of decreased pigmentation is different in pityriasis versicolor (Chapter 12). In addition, collections of naevus cells occur in benign melanocytic naevi and in the various types of malignant melanoma.