Objectives: Internationally, public health strategies encourage health care professionals to deliver opportunistic behaviour change interventions. The present study: (1) examines the barriers and enablers to delivering interventions during routine consultations, and (2) provides recommendations for the design of interventions to increase delivery of opportunistic behaviour change interventions. Design: Qualitative interview study. Methods: Twenty-eight semi-structured interviews were conducted with patient-facing health care professionals. The Behaviour Change Wheel informed a framework analysis in which findings were mapped onto the Theoretical Domains Framework (TDF). Intervention functions and behaviour change techniques (BCTs) targeting each TDF domain were identified. Results: Health care professionals understood the importance of opportunistic behaviour change interventions (beliefs about consequences), but were sceptical about their capabilities to facilitate behaviour change with patients (beliefs about capabilities). Some clinicians were unwilling to discuss behaviours perceived as unrelated to the patient's visit (social/professional role and identity). Discipline-specific tasks were prioritized, and delivering interventions was perceived as psychologically burdensome. One-to-one contact was favoured over busy hospital settings (environmental context and resources). Seven intervention functions (training, restriction, environmental restructuring, enablement, education, persuasion, and modelling) and eight BCT groupings (antecedents, associations, comparison of outcomes, covert learning, feedback and monitoring, natural consequences, reward and threat, and self-belief) were identified. Conclusions: Across disciplines, health care professionals see the value of opportunistic behaviour change interventions. Barriers related to workload, the clinical environment, competence, and perceptions of the health care professional role must be addressed, using appropriate intervention functions and BCTs, in order to support health care professionals to increase the delivery of interventions in routine practice. Statement of contribution What is already known on this subject? Brief, opportunistic interventions can be a cost effective way of addressing population health problems. Public health policies compel health care professionals to deliver behaviour change interventions opportunistically. Health care professionals do not always deliver interventions opportunistically during routine medical consultations; however the barriers and enablers are currently unclear. What does this study add? This is the first study to examine cross-disciplinary barriers and enablers to delivering opportunistic behaviour change interventions. Across diverse professional groups, working in different medical professions, participants saw the value of delivering opportunistic behaviour change interventions. Targeting key theoretical domains that are shared across professional groups may be useful for increasing the delivery of opportunistic behaviour change interventions.