Abstract Disorders of the mouth, whether as a consequence of primary disease, systemic disease or as a consequence of treatment, may be encountered across most medical specialities. Whilst recurrent aphthous ulceration represents the most common primary oral disease, oral lesions may indicate active systemic disease at less accessible sites. Moreover, when oral disease is present, it may have a detrimental impact on quality of life. For these reasons it is important to be aware of the more common lesions affecting the oral cavity and to formulate a differential diagnosis appropriate for each lesion. In this article, we address the more common disorders seen in clinical practice. Distinction is made between primary oral disease and systemic disease with oral manifestations. Moreover, we have attempted to categorize systemic diseases affecting the oral cavity into the medical specialties where they are likely to be encountered and for ease of reference for the general reader. Finally, we include a table of commonly used therapeutic regimens for oral diseases, summarizing their mode of action and indications.