Acute generalized exanthematous pustulosis (AGEP) is a significant adverse cutaneous reaction, most often provoked by drugs and acute infections. The recognition of AGEP is important, in order to avoid confusion with a systemic infection and consequently to avoid incorrect treatment. The clinical hallmark is the presence of multiple disseminated sterile pustules on an erythematous background, associated with fever and a massive neutrophilia and sometimes eosinophilia. The disease is characterised by an acute onset and a spontaneous resolution within 2 weeks. The involvement of drug-specific T cells in the pathomechanism can be confirmed by positive skin patch tests and lymphocyte transformation tests. In this review, we highlight the main clinical, pathophysiological and diagnostic aspects of this peculiar form of drug allergy.