Breakthrough pain is a prevalent cancer pain syndrome, and research is needed to identify more effective interventions to manage it. A validated tool to assess breakthrough pain in a standard and reliable manner is urgently needed to support the conduct of clinical trials in breakthrough pain. To address this need, we developed a breakthrough pain assessment tool for research purposes. The current study was undertaken to gather validity evidence for this breakthrough pain assessment tool, using a Delphi process involving an expert panel review, followed by a think-aloud process involving patients with cancer-related breakthrough pain. Two expert panels were formed: a national panel (within Canada; n=16) and an international panel (including experts from North America, UK, Europe, the Middle East, Australia, and New Zealand; n=22). Each panel participated in one anonymous survey round. Response rates were 56% (national panel) and 73% (international panel). The Delphi process revealed substantial consensus on the content of the tool, which increased between rounds of review. The overall level of agreement with the tool, averaged over the four evaluated aspects of all items, was 80% among national panelists and 88% among international panelists. Nine patients completed the think-aloud study. They were able to understand and complete the tool and provided specific direction on its improvement. The validity evidence gathered in this study suggests the Alberta Breakthrough Pain Assessment Tool is conceptually grounded and is understandable by patients and clinicians. Further validation of this tool as an assessment measure within clinical trials research is warranted.