Changes in the appearance of actinic keratosis (AK) suggest progression to invasive squamous cell carcinoma (SCC), though some dermatologists and dermatopathologists consider AK to be SCC in situ. Actinic keratosis is an indicator of cumulative UV exposure and the initial lesion in the majority of invasive cutaneous SCCs. The development of SCC on sun-damaged skin is a gradual process; however, most AK lesions do not progress to invasive SCC and it currently is not possible to clinically or histopathologically determine which AK lesions will progress to SCC. Presently there is insufficient evidence to support the concept that AK is frank SCC. Although the rate of progression over time remains to be determined by large prospective studies, AK is a marker for an increased rate of nonmelanoma skin cancer (NMSC), even in the absence of specific lesion progression. Nevertheless, the risk for progression of AK to invasive SCC with the potential for metastasis provides the rationale for treatment, and AK lesions should be treated with lesion- or field-directed therapy or with a combined approach when indicated. We discuss the implications for treatment and review a variety of treatment options.