BACKGROUND: Photoallergic contact dermatitis (PACD) presents in patients after certain exogenous agents come into contact with the skin in the presence of ultraviolet and/or visible light. The best method currently available for investigating PACD is photopatch testing. However, photopatch testing as an investigation is under-used by clinicians, and therefore PACD may go undetected in many patients. PURPOSE: To highlight the importance of PACD and photopatch testing when investigating patients with a photo-exposed site dermatosis. METHOD: A comprehensive review of the available literature relating to PACD and photopatch testing. RESULTS: Experimental evidence suggests that PACD is a delayed type hypersensitivity reaction. Various agents have been historically shown to cause PACD, but currently the most common photosensitizers are sunscreens and topical non-steroidal anti-inflammatory drugs. Photopatch testing has in the past been subject to differing methodologies; however, a European consensus methodology now exists and should allow a greater comparison of results across centres. As chemical, pharmaceutical, and cosmetic industries produce new agents, photopatch testing of such agents in humans before release in the marketplace may prevent widespread contact with potent photosensitizers. It will also be important for ongoing multi-centre studies of existing agents to be conducted in order to keep the photopatch test batteries used by clinicians investigating PACD up to date.