The term Erythema Multiforme (EM) include actually a wide range of clinical expressions, from exclusive oral erosions (Oral EM) to mucocutaneous lesions (EM Minor), sometimes with severe involvement of multiple mucosal membrane (EM major, Stevens-Johnson syndrome [SJS]) or with involvement of a large area of the total body surface (toxic epidermal necrolysis [TEN]). However, this terminology is not worldwide accepted and often the various clinical categories show some overlapping features. Among the great number of suspected etiological factors, herpes simplex virus is involved in many cases of EM minor whereas SJS and TEN are caused in 80% of cases by systemic drugs, mainly by anticonvulsivants, sulfonamides, nonsteroidal anti-inflammatory drugs and antibiotics. Several oral EM seem idiopathic, but data on this topic are very few. There is no specific or consistent microscopic and immunopathologic pattern of EM and the diagnosis should be done by excluding other similar diseases. The treatment include the use of antivirals for EM minor, mainly if recurrent, and of immunosuppressants (especially systemic corticosteroids) for SJS. TEN patients require adequate supportive care and often they have to be treated in emergency departments. Finally, patients with exclusive oral lesions may be treated with both topical and systemical corticosteroids.