Publisher Summary This chapter presents a comparison between psoriasis and cancer and provides annotated diagrams that depict, describe, compare, and contrast the cycling⇆noncycling cell proliferative transitions as they apply to the problems of psoriasis and cancer in man. The major clinical aspects of psoriasis—that is, the original manifestation, remission, and relapse of active psoriasis—are explained in terms of recruitment and return of germinative epidermal cells to the cycling and noncycling states. Psoriasis is a disease of epidermal hyperplasia resulting in a faster epidermal cell transit to the stratum corneum and to excessive scaling, which is the main clinical disturbance in psoriasis. Cancer is a disease of the transformation of normal cells in a heritable manner and the establishment of invasive primary, secondary, and disseminated tumor growths. The chapter discusses the psoriatic proliferative responses and tumor proliferative responses to therapy.