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The vomiting centre can be stimulated either directly or indirectly via the chemoreceptor trigger zone (CRTZ). Disease or irritation of the gastrointestinal (GI) tract, abdominal organs or peritoneum can directly stimulate the vomiting centre, as can cerebral diseases. Blood-borne substances (such as toxins or drugs) and neurological input from the vestibular nucleus cause dopamine release in the CRTZ and indirect stimulation of the vomiting centre. Once the vomiting centre is stimulated a set of reflex actions are coordinated to cause the active expulsion of gastric contents from the mouth. The clinical importance of vomiting stems from its association with a large and varied group of diseases, and the potentially life-threatening consequences of vomiting per se, such as fluid and electrolyte depletion, acid-base derangement, aspiration pneumonia and oesophagitis. This chapter examines Clinical features; Differential diagnosis; and Diagnosis.