Introduction: The expanding role of physiotherapists, with increasing use of services such as self-referral, means that demonstrating an ability to understanding and ask red-flag questions appropriately has never been more important. The present study investigated how physiotherapists define common red flags, how they ask red-flag questions, which red flags they routinely record and the importance that therapists attribute to individual red-flags. Methods: In this qualitative study, an online questionnaire survey was circulated to physiotherapists using various UK special interests networks. A total of 98 physiotherapists participated in the study, the majority of whom worked exclusively in the National Health Service. Results: In general, clinicians defined red flags for back pain in line with guidelines but there was little consensus on how therapists asked patients about red flags. Questioning often included undefined terminology and therapists not asking some red-flag questions. Additionally, the more recently recognized flags (vague, non-specific lower-leg symptoms; band-like trunk pain; decreased mobility) had less importance attached to them and were recorded less frequently. Conclusions: If only certain red flags are being assessed, this may put patients at risk of having serious spinal pathologies going undetected. Thus, strategies encouraging therapists to ask all red-flag questions may be needed. The importance of the more recently recognized red flags may need to be emphasized to clinicians. Finally, the inconsistent way in which the red-flag questions were asked highlights a potential practical barrier to translating red-flag knowledge into clinical practice. There is a need to build on this work, using in-depth qualitative interviews, to gain a deeper understanding of how therapists understand and apply the red flags commonly used in back pain assessment.