Purpose: The Judgement and Decision Making Performance in Practice (JDM-PiP) assessment tool has been developed with the aim of assisting nursing staff to critically reflect on the judgements and decisions they make in clinical practice. Method: Few practical assessment tools are available to permit clinicians to analyse their judgements and decisions. To help address this gap, an idiographic assessment tool was devised and tested over a three-year period using successive cohorts of experienced nurses (N=95) registered on a specialised postgraduate module in clinical judgement and decision making. The majority of course participants were working in mental health nursing. A review of the research literature identified a set of cognitive and social / organisational factors relevant to judgement and decision making. These factors were integrated in a conceptual model and operationalised as individual items for a clinician to consider in respect of a clinical case self-selected from practice. When used in an educational context, verbal or written feedback is provided by an assessor to some or all responses, with the aim of stimulating critical reflection. Results: The cases presented by respondents varied in respect of the clinical challenges posed (e.g., an unusual constellation of symptoms, miscommunication between health professionals, a problem with several plausible causes), and were generally conceptualised using a biopsychosocial model. Several difficulties were experienced by respondents in articulating the judgement and decision making process (e.g., categorising information at different levels of abstraction, mapping subjective judgements onto quantitative scales, discussing patient preferences). Respondents appraised the JDM-PiP positively in qualitative and quantitative evaluations as a valuable means to articulate and analyse their clinical judgements and decisions. Conclusions: The development work conducted on the JDM-PiP demonstrates its utility in an educational context. By accommodating clinical, interpersonal and organisational factors, it provides insights on clinical knowledge and the organisation of care. Further applications of the JDM PiP may be explored, including integration in professional development programmes and use as a research tool. Considering the generic nature of many aspects of judgement and decision making, the JDM PiP may also be used with several other health care professions.