The purpose of this article was to review the frequency, distribution, and determinants of actinic keratoses (AKs) and squamous cell carcinoma (SCC). A review of the literature was done. AKs are extremely common lesions on the sun-exposed skin of Caucasian persons. The most important risk factors are a combination of genetic propensity ("fair skin phenotype") and cumulative sun exposure. Their prevalence increases with advancing age. The epidemiology of SCC is virtually the same, but the lesions occur most often on the head rather than on the upper extremities where most AKs are located. AKs are the most important risk factor identifying those most predisposed to the development of an SCC. AKs are a reliable marker for those people most predisposed to development of an invasive SCC. In addition, AKs are probably an early stage in a biologic continuum that culminates in SCC. However, there are many more AKs than SCCs, and it is difficult to predict exactly which lesions will progress to invasive cancer.