The usual management of cancer related breakthrough pain is with supplemental doses of analgesics (commonly opioids) at a dose proportional to the total around-the-clock opioid dose. The aim of this review, undertaken as part of a European Palliative Care Research Collaborative (EPCRC) project, to update the EAPC guidelines on opioid analgesics in cancer pain was to determine the evidence for the utility of opioids in the management of breakthrough pain in patients with cancer. Randomized controlled trials of opioids used as rescue medication were identified using electronic search strategies. Outcome measures sought were reduction in pain intensity measured by an appropriate scale, adverse effects, attrition, and patient satisfaction. The date of the final search was 31 July 2009. Eight studies (790 patients) met the inclusion criteria. Most studies investigated rescue medication delivery via the buccal or nasal transmucosal routes. Intravenous morphine has been compared with the transmucosal route and the two found to be effective. The oral route has not been formally tested although found to be an inferior comparator in one study. Most studies showed no meaningful relationship between the effective dose of transmucosal opioid and the around-the-clock scheduled medication or the previous rescue medication, although one study found a fixed proportion of either intravenous morphine or transmucosal fentanyl to be efficacious.