Oral mucosal diseases refer to diseases that occur in the oral mucosa and soft tissues, except tumors. There are many types of oral mucosal diseases, with a low prevalence, a variety of lesions, and complex causes. Some diseases are closely related to systemic factors, and a holistic concept should be maintained during diagnosis and treatment. Diseases in which the normal color, shape, integrity, and function of the mucosa in a certain part of the oral cavity are changed. There are many types of lesions, which can be combined into complex and diverse lesions. Some systemic diseases also manifest themselves on the oral mucosa, and some oral manifestations can serve as a basis or clue for the diagnosis of systemic diseases.