Breakthrough pain in children with cancer is an exacerbation of severe pain that occurs over a background of otherwise controlled pain. There are no randomized controlled trials in the management of breakthrough pain in children with cancer, and limited data and considerable experience indicate that breakthrough pain in this pediatric patient group is common, underassessed, and undertreated. An ideal therapeutic agent would be rapid in onset, have a relatively short duration, and would be easy to administer. A less effective pharmacologic strategy would be increasing a patient’s dose of scheduled opioids, because this may increase the risk of oversedation. The most common and effective strategy seems to be multimodal analgesia that includes an immediate-release opioid (eg, morphine, fentanyl, hydromorphone, or diamorphine) administered intravenously by a patient-controlled analgesia pump, ensuring an onset of analgesic action within minutes. Intranasal fentanyl (or hydromorphone) may be an alternative, but no pediatric data have been published yet for commercially available fentanyl transmucosal application systems (ie, sublingual tablets/spray, buccal lozenge/tablet/film, and nasal spray), and these products cannot yet be recommended for use with children with cancer and breakthrough pain. The aim of this paper was to emphasize the dearth of available information on treatment of breakthrough pain in pediatric cancer patients, to describe the treatment protocols we currently recommend based on clinical experience, and to suggest future research on this very important and under-researched topic.