This guide provides practical guidance for applying the Theoretical Domains Framework (TDF) to assess implementation problems and support intervention design. The TDF was developed by behavioural scientists and implementation researchers to identify determinants of behaviour, and has been cited in over 800 peer-reviewed publications. It is a theoretical framework that synthesises 128 theoretical constructs from 33 theories relevant to implementation. The TDF has been used in a wide range of healthcare settings and clinical behaviours, and has been the basis of studies with various objectives and designs. The guide outlines the steps for applying the TDF, including selecting and specifying a target behaviour, selecting study design, deciding the sampling strategy, developing an interview schedule, and collecting and analysing data. It also discusses the use of the TDF in questionnaire studies and systematic reviews. The guide highlights the importance of using qualitative approaches, such as interviews and focus groups, to explore barriers and facilitators to behaviour change. It also discusses the challenges of using the TDF, including the need for training and resources, and the importance of triangulating data from multiple sources. The guide concludes that the TDF provides a theoretical basis for implementation studies, good coverage of potential reasons for slow diffusion of evidence into practice, and a method for progressing from theory-based investigation to intervention. The TDF has been applied in various areas, including healthcare, and has been used to understand influences on behaviours such as patient behaviours, recycling behaviours, and others. The guide is intended to assist the implementation community in applying the TDF to achieve implementation objectives.This guide provides practical guidance for applying the Theoretical Domains Framework (TDF) to assess implementation problems and support intervention design. The TDF was developed by behavioural scientists and implementation researchers to identify determinants of behaviour, and has been cited in over 800 peer-reviewed publications. It is a theoretical framework that synthesises 128 theoretical constructs from 33 theories relevant to implementation. The TDF has been used in a wide range of healthcare settings and clinical behaviours, and has been the basis of studies with various objectives and designs. The guide outlines the steps for applying the TDF, including selecting and specifying a target behaviour, selecting study design, deciding the sampling strategy, developing an interview schedule, and collecting and analysing data. It also discusses the use of the TDF in questionnaire studies and systematic reviews. The guide highlights the importance of using qualitative approaches, such as interviews and focus groups, to explore barriers and facilitators to behaviour change. It also discusses the challenges of using the TDF, including the need for training and resources, and the importance of triangulating data from multiple sources. The guide concludes that the TDF provides a theoretical basis for implementation studies, good coverage of potential reasons for slow diffusion of evidence into practice, and a method for progressing from theory-based investigation to intervention. The TDF has been applied in various areas, including healthcare, and has been used to understand influences on behaviours such as patient behaviours, recycling behaviours, and others. The guide is intended to assist the implementation community in applying the TDF to achieve implementation objectives.