Epidermis provides the cutaneous barrier to the external environment and undergoes a continual process of proliferative self-renewal, with human epidermis undergoing complete turnover approximately 1,000 times in a lifetime. Recent work suggests that this ongoing proliferative replenishment of epidermal cells depends, in part, on continual signals for cell division and survival transmitted by the Ras/Erk MAPK pathway. Such constant cell proliferation, however, requires tight regulation to avoid the uncontrolled tissue expansion characteristic of epidermal neoplasia. Recent studies provide new insight into Ras/Erk MAPK pathway function in the control of normal skin development and homeostasis as well as how its deregulation promotes epidermal tumorigenesis.