Breakthrough pain is the relatively new term applied to describe acute episodes of severe pain superimposed upon a background of otherwise well controlled chronic pain. Surveys of patients with cancer who have pain suggest that breakthrough pain is very common, affecting the majority of patients several times a day on average. The cause of breakthrough pain is tumour related in most cases, and is fairly evenly distributed among somatic, visceral, neuropathic and mixed aetiologies. Not unexpectedly, the incidence and prevalence of breakthrough pain increase with advancing disease. About half of all breakthrough pain episodes are attributable to a specific inciting event or activity, and so these types of pains have been categorised as incident pain. Another subcategory of breakthrough pain is end-of-dose failure, caused by pain that ‘breaks through’ around-the-clock medication because of declining blood concentrations of analgesic near the end of the administration interval. The remainder of breakthrough pain episodes, classified as spontaneous pain, are unpredictable in occurrence.The treatment of breakthrough pain needs to be preventative and active. Immediate release opioid analgesics are the mainstay of therapy in most cases. Few techniques and interventions have consistently been shown to ameliorate breakthrough pain, with the exception of oral transmucosal fentanyl citrate. This novel formulation of the potent opioid fentanyl is a hard candied matrix on a handle that is dissolved in the patient's mouth yielding a rapid onset of symptom relief. Although safety and efficacy have been well demonstrated in controlled clinical trials, this product has not been available for prescription long enough to determine its overall impact on this very common distressing symptom which is experienced by most cancer patients.