Detailed questioning and careful analysis of a skin eruption usually lead to the diagnosis of photodermatitis on two major criteria: the eruption is clearly related to a period of exposure to sunlight, and it is localized to, or predominant on, uncovered regions. However, the differential diagnosis with contact eczema of the face or air borne dermatitis due to irritation or allergy is sometimes difficult. The skin lesions of photosensitization result from two physiopathological mechanisms: phototoxicity, a photo-chemical reaction, and photoallergy, an immunoallergic reaction to an antigen formed in the skin after light irradiation. In the evaluation of photodermatitis, photobiological exploration ranks first as it confirms the diagnosis, informs on the mechanism and identifies a possible photosensitizing agent. It is useful in the nosological classification of light eruptions and permits to adjust the treatment according to the responsible photons. Depending on the presumed nature of the photodermatitis studied, various phototests are performed, including Saidman's tests, photochallenge tests, photopatch tests and phototest after reintroduction of a drug.