Erythema multiforme (EM), in its modern definition, is an acute, self-limited syndrome with distinctive skin lesions with or without mucosal lesions. Use of the terminology "EM minor" and "EM major" is a reasonable approach to separating the classical mild cutaneous syndrome, as described by Hebra (EM minor), from the usually more severe syndrome, with marked mucosal damage, as described by Stevens and Johnson (EM major). Until objective markers for EM are available, we propose preliminary diagnostic criteria for EM, based primarily on clinical features. Although hundreds of factors have been reported to cause EM, only a limited number are reasonably well documented as possible precipitating agents. Recurrent herpes simplex is an important etiologic factor in EM minor, while mycoplasmal infections and drugs may be associated with EM major. Progress in the understanding of EM will require careful attention to definition and diagnostic criteria and identification of distinct clinical and etiologic subsets within the spectrum of EM.