Ulcers commonly occur in the oral cavity, their main symptom being pain. There are different ways to classify oral ulcers. The most widely accepted form divides them into acute ulcers –sudden onset and short lasting– and chronic ulcers –insidious onset and long lasting–. Commonest acute oral ulcers include traumatic ulcer, recurrent aphthous stomatitis, viral and bacterial infections and necrotizing sialometaplasia. On the other hand, oral lichen planus, oral cancer, benign mucous membrane pemphigoid, pemphigus and drug-induced ulcers belong to the group of chronic oral ulcers. It is very important to make a proper differential diagnosis in order to establish the appropriate treatment for each pathology.