 Actinic keratoses AK are common precancerous skin lesions that arise on sun-damaged skin. Treatment is aimed at preventing progression to cutaneous squamous cell carcinoma SCC. First-line therapy for clinically apparent lesions includes cryotherapy and curettage; and field therapy options are topical 5-fluorouracil, imiquimod, ingenol mebutate, and photodynamic therapy PDT. PDT involves the topical application of aminolevulinic acid ALA, or one of its derivatives, as a photosensitizing agent. In response, rapidly proliferating, dysplastic cells preferentially accumulate protoporphyrin IX PpIX. When PpIX is activated by blue or red light, singlet oxygen species are produced, resulting in cell death. PDT is beneficial due to its brief treatment course and efficacy in clearing AK. However, its main drawbacks are the adverse effects of pain, burning, pruritus, erythema, crusting, and inflammation associated with treatment. While conventional PDT uses red or blue artificial light to activate a high concentration of accumulated protoporphyrins, daylight PDT uses natural daylight to activate lower levels of protoporphyrins in a continuous manner. Daylight PDT, when compared with conventional PDT, has been associated with significantly less pain while achieving comparable efficacy for the treatment of AK. Daylight PDT is also more cost-effective and reduces the amount of time spent in clinic. Previous randomized studies comparing daylight PDT with conventional PDT have largely used methyl-aminolevulinate as the photosensitizer, have been intra-individual comparative studies, and have been performed in Nordic countries. Because the effective light dose from natural daylight depends on geographic location and seasonal and weather changes, randomized trials in different geographic and environmental conditions are of interest. The proposed randomized clinical trial investigates the tolerability and efficacy of daylight ALA-PDT for the treatment of AK in San Francisco for the first time; subjects will be randomized to various treatment arms, as opposed to previous split-face and intra-individual studies. 