Clinically, actinic keratoses (solar keratoses) are keratotic lesions that occur on chronically sunlight-exposed skin. Histologically, they are epidermal tumors. Presenting with a spectrum of clinical features, actinic keratoses are believed to be predictors and precursors of invasive squamous cell carcinoma. This, the first of a two-part overview, examines the etiology and epidemiology of actinic keratoses, their clinical and histological features and their role in the development of nonmelanoma skin cancer.