Non-melanoma skin cancer patients are believed to be at high risk of developing new skin malignancies; however, relatively few studies have actually examined this. Non-melanoma skin cancers, although generally not fatal, are responsible for significant morbidity and, if left untreated, can cause serious disfigurement and, in rare instances, death. Part of the difficulty in studying these tumors is that they are not routinely followed as rigorously as other cancers, and are excluded from most cancer registries. Prior studies have often included patients from a single dermatology practice of those treated with one type of therapy (e.g., Moh's surgery). Some studies did not entail active follow-up of patients but relied on reports to a registry or subsequent visits initiated by the patient or their physician. Other studies have been based solely on patients with complete follow-up information, excluding those who died or were lost to follow-up. Nearly all studies collected only a limited amount of information on each patient. In an attempt to overcome these limitations, we examined subsequent skin cancer occurrence among a group of patients followed as part of the Skin Cancer Prevention Study. In the following, we highlight some of the methodologic issues concerning the study of subsequent basal cell and squamous cell skin cancer risk, and the contribution of our work, along with that of others, in exploring clinical and etiologic questions pertaining to the occurrence of these malignancies.