Photodermatosis is an abnormal skin inflammatory reaction to light. The major classifications of photodermatoses are idiopathic photodermatoses, photodermatoses due to exogenous or endogenous agents, photo-exacerbated dermatoses, and photosensitive genodermatoses. In this chapter, we focus on idiopathic photodermatoses and drug-related photodermatoses and emphasize on the epidemiology and immunogenetic backgrounds. Idiopathic photodermatoses, a spectrum of diseases with abnormal responses to ultraviolet radiation (UVR), include polymorphous light eruption, actinic prurigo, hydroa vacciniforme, chronic actinic dermatitis, and solar urticaria. Young people are more susceptible to most idiopathic photodermatoses except for chronic actinic dermatitis. Interestingly, idiopathic photodermatoses exhibit different characteristics between Caucasians and Asians. For example, the average age of Asian actinic prurigo patients is older than that of Caucasians in which genetic backgrounds or Fitzpatrick skin type might play a role. Drug-induced photodermatoses can be classified into phototoxic and photoallergic drug reactions. Certain drug-induced photodermatoses may mimic other dermatoses. For instance, drug-induced lupus erythematosus (LE) should be considered if an old man is diagnosed with LE but had a poor response to standard treatments.