Background: Many health professionals working with teenage and young adult cancer patients (TYA-HPs) do not provide advice on physical activity, dietary intake, smoking cessation, and alcohol consumption as part of routine cancer care. Objective: The aim of this study was to understand TYA-HPs' perspectives on the provision of health behavior advice and preferences on an intervention to help develop their health promotion skills.Methods: In-depth, semistructured interviews were conducted with 26 TYA-HPs ( 12nurses, 8 clinicians, and 6 allied health professionals) whose average time working with teenage and young adult (TYA) cancer patients was 8 years. Each interview followed the same semistructured guide, which was based upon constructs of the COM-B model of behavior change (capability, physical opportunity, social opportunity, reflective motivation, and automatic motivation), transcribed verbatim, and analyzed using the Framework analysis. Results: Overall, TYA-HPs recognized the value and importance of promoting health behaviors but felt that their capability to provide health behavior advice was limited by the availability of, and access to, good-quality evidence linking health behavior to cancer outcomes. The TYA-HPs expressed confusion over professional