Aphthous ulcers are painful ulcerations located on the mucous membrane, generally in the mouth, less often in the genital area. Three clinical forms of aphthous ulcers have been described: minor aphthous ulcers, herpetiform aphthous ulcers and major aphthous ulcers. Many other conditions presenting with oral bullous or vesiculous lesions orulcerations and erosions can be mistaken for aphthous ulcers. Currently, treatment of aphthous ulcers is palliative and symptomatic. Topical treatments (topical anesthetics, topical steroids and sucralfate) are the first line therapy. Recurrent aphthous stomatitis (RAS) is defined by the recurrence of oral aphthous ulcers at least 4 times per year. RAS is often idiopathic but can be associated with gastro-intestinal diseases (i.e. celiac disease, inflammatory bowel diseases), nutritional deficiencies (iron, folates...), immune disorders (HIV infection, neutropenia) and rare syndromes. Behçet's disease is a chronic, inflammatory, disease whose main clinical feature is recurrent bipolar aphthosis. Colchicine associated with topical treatments constitutes a suitable treatment of most RAS. Thalidomide is the most effective treatment of RAS but its use is limited by frequent adverse effects. Oral ulcers can be related to a wide range of conditions that constitute the differential diagnoses of aphthous ulcers. Oral ulcers are classified into three main groups: acute ulcers with abrupt onset and short duration, recurrent ulcers (mainly due to postherpetic erythema multiforme) and chronic ulcers (with slow onset and insidious progression). Acute oral ulcers are due to trauma, bacterial infections (including acute necrotizing ulcerative gingivitis), deep fungal infection, gastro-intestinal (namely inflammatory bowel disease) or systemic diseases. Chronic oral ulcers may be drug-induced, or due to benign or malignant tumors. Every oral solitary chronic ulcer should be biopsied to rule out squamous cell carcinoma. A solitary palatal ulcer can be related with necrotizing sialometaplasia.