Rationale, aims and objectives: Motivational Interviewing (MI) is an individual-level approach to behaviour change that has been evaluated in over 600 randomised clinical trials across multiple settings. Increasingly, research efforts focus on how MI works and how it can best be integrated into public health and clinical programmes. As the application of MI expands, a key integration challenge involves specifying the focus of a conversation such that a practitioner might ignite and intensify a patient?s internal motivation for change related to that focus. At present, this challenge is poorly conceptualised. We aimed to clarify the construct of ?focusing? and to develop a practical tool that can be used to develop and assess practitioner competence. Method: First we reviewed validated MI measures to elucidate current conceptualisations of focusing. Second, we identified practitioner speech acts that led to topic transitions. We then drafted the first version of MIFI. A gold standard rater, together with one expert MI and 3 non-expert MI raters, each used MIFI to coded 20 audio recordings from a feasibility study of MI and breastfeeding maintenance (n=170 observations). Internal consistency and inter-rater reliability analyses were conducted. Results: Published MI measures include ?focusing? as a strategy to agree a target change or to hold attention on that change target. We observed practitioners create or shift focus using 4 skills: questions, listening statements, giving information or meta-statements. Moderate to strong correlations were demonstrated between 4 of 5 global measures on the MIFI. Reliability estimates were good to excellent overall (5 coder ICCs>0.65), fair to excellent for the non-expert coding group (ICCs>0.55) and for the best coding pair (MI expert and non-expert ICCs >0.52). Conclusion: We offer conceptual clarity about focusing in MI and have developed a tool to train practitioners in ?focusing? when integrating MI into healthcare and public health interventions.