Skin cancer is the most frequent malignancy in organ transplant recipients, 95% of which are nonmelanoma skin cancer, especially squamous cell and basal cell carcinomas. This paper also discusses the incidence of other tumors (eg, melanoma, Merkel cell carcinoma, and Kaposi sarcoma) that are also increased in organ transplant patients compared to the general population. Part I of this two-part series describes the latest data concerning the epidemiologic and pathogenic aspects of nonmelanoma skin cancer development in solid organ transplant recipients. This review also highlights the concept of "field cancerization," represented by extensive areas of actinic damage and epidermal dysplasia, which accounts for increased risk of aggressive skin cancer development in susceptible patients. Copyright © 2010 American Academy of Dermatology, Inc. Published by Mosby, Inc. All rights reserved.