Introduction: Clinical reasoning is an essential part of medical practice. Training medical students to reason competently is, therefore, an essential skill for clinical teachers. Ongoing debate over what clinical reasoning entails and difficulty explicitly teaching and assessing it makes this a challenging task. This study explored clinical teachers' understanding of the concept of clinical reasoning. Methods: Nine experienced clinical teachers participated in semi-structured interviews about clinical reasoning, exploring concepts, experiences, teaching and assessment. Interviews were transcribed and analysed thematically. Results: Ten key themes were identified in relation to participants' understanding of clinical reasoning. These include the findings that clinical reasoning is: essential to medical practice, goal oriented, an applied cognitive process and an unconscious process. Clinical reasoning has several requirements, including knowledge, communication skills, experience and reflection. Participants reported that clinical reasoning is difficult, perhaps impossible, to teach. Conclusions: In this qualitative study, clinical teachers concurred with many of the characteristics of clinical reasoning as it is understood in the literature, but they also challenged assumptions made in medical education research regarding the conceptualisation of clinical reasoning. This has implications for teaching, assessment, student selection and professional development.