This guide provides practical guidance on using the Theoretical Domains Framework (TDF) to assess implementation problems and support intervention design. The TDF, developed by behavioral scientists and implementation researchers, is a comprehensive, theory-informed approach to identify determinants of behavior change. The guide covers the rationale for using a theoretical approach, the development and structure of the TDF, and methodological considerations for applying it. It includes examples from the literature to illustrate methods and practical considerations, focusing on qualitative approaches such as interviews and focus groups. The guide aims to assist researchers and practitioners in understanding and addressing implementation challenges by providing detailed steps for selecting target behaviors, designing studies, sampling, developing interview schedules, collecting and analyzing data, and reporting findings. It also discusses areas for further development, such as triangulating data from multiple sources and designing interventions based on TDF-based problem analysis.This guide provides practical guidance on using the Theoretical Domains Framework (TDF) to assess implementation problems and support intervention design. The TDF, developed by behavioral scientists and implementation researchers, is a comprehensive, theory-informed approach to identify determinants of behavior change. The guide covers the rationale for using a theoretical approach, the development and structure of the TDF, and methodological considerations for applying it. It includes examples from the literature to illustrate methods and practical considerations, focusing on qualitative approaches such as interviews and focus groups. The guide aims to assist researchers and practitioners in understanding and addressing implementation challenges by providing detailed steps for selecting target behaviors, designing studies, sampling, developing interview schedules, collecting and analyzing data, and reporting findings. It also discusses areas for further development, such as triangulating data from multiple sources and designing interventions based on TDF-based problem analysis.