**Abstract**

Apart from dental and periodontal diseases, oral mucosal lesions are also frequently encountered by both general dentists and dental specialists in outpatient clinics. Although these soft tissue lesions may only reveal a localized issue, sometimes they may be the only sign of a more serious underlying systemic condition. Thus, oral ulcerations pose a unique diagnostic challenge for healthcare providers and should be cautiously handled when they last for more than two weeks, even after any possible traumatic etiologies are eliminated. There are many different classifications regarding oral ulcerations based on their etiologic or clinical features. In order to provide a logical and simple stepwise guidance to accurate diagnosis, this chapter will categorize and explain these lesions based on their clinical properties.

**Keywords:** oral ulcer, oral mucosa, oral disease, aphthous stomatitis
