Aims: The aim of this narrative synthesis was to explore the necessary and sufficient conditions required to define moral distress. Background: Moral distress is said to occur when one has made a moral judgement but is unable to act upon it. However, problems with this narrow conception have led to multiple redefinitions in the empirical and conceptual literature. As a consequence, much of the research exploring moral distress has lacked conceptual clarity, complicating attempts to study the phenomenon. Design: Systematic literature review and narrative synthesis (November 2015–March 2016). Data sources: Ovid MEDLINE^® In-Process & Other Non-Indexed Citations 1946–Present, PsycINFO^® 1967–Present, CINAHL^® Plus 1937–Present, EMBASE 1974–24 February 2016, British Nursing Index 1994–Present, Social Care Online, Social Policy and Practice Database (1890–Present), ERIC (EBSCO) 1966–Present and Education Abstracts. Review methods: Literature relating to moral distress was systematically retrieved and subjected to relevance assessment. Narrative synthesis was the overarching framework that guided quality assessment, data analysis and synthesis. Results: In all, 152 papers underwent initial data extraction and 34 were chosen for inclusion in the narrative synthesis based on both quality and relevance. Analysis revealed different proposed conditions for the occurrence of moral distress: moral judgement, psychological and physical effects, moral dilemmas, moral uncertainty, external and internal constraints and threats to moral integrity. Conclusion: We suggest the combination of (1) the experience of a moral event, (2) the experience of ‘psychological distress’ and (3) a direct causal relation between (1) and (2) together are necessary and sufficient conditions for moral distress.