The mouth has often been looked at as a window into the body, and this is no truer than in oral medicine. The oral mucosa is a highly adapted and robust tissue, which, at the same time, can be very susceptible to changes in homeostasis. Dysregulation of the immune system, alterations in cellular signalling pathways, or insult from exogenous stimuli can lead to an array of weird and wonderful oral lesions. Clinicians today are likely to see changes to the oral mucosa on a regular basis— from common ulcers, bony lumps, or white patches to more exotic pigmented lesions or unusual lumps. It is therefore vital to have good basic knowledge of common conditions and be able to identify lesions that need urgent referral and treatment. It is important to take a thorough medical history, as many oral symptoms can be associated with systemic conditions or changes in medication. A temporal link can be a good indicator of causation from medication changes. Having a strategic method of constructing a list of differential diagnoses, such as the surgical sieve, can be a great aide- memoire to ensure all the pertinent questions and investigations have been completed. However, it must be noted that many conditions cannot be accurately diagnosed without histological examination, and therefore, referral for specialist input is commonplace. Oral medicine can be a tricky discipline, fraught with challenging patients to manage, particularly those with chronic conditions. Conversely, the diagnostic challenges make for a thoroughly rewarding and stimulating discipline. The questions in this chapter will test your knowledge of disease symptoms, links to medical conditions, and patient management. Key topics include: ● Patient assessment and diagnosis ● Investigations ● Basic histology ● Infections (bacterial, viral, and fungal) ● Ulcers ● Soft tissue swelling ● Bony lumps ● Systemic conditions ● White, red, pigmented, and mixed patches ● Oral cancer ● Pharmacology.