In the UK, there is growing concern regarding the increasing prevalence of social, emotional and mental health (SEMH) difficulties experienced by children and young people. Using thematic analysis, this study sought ‘to determine how a sample of headteachers (HTs) define, identify and recognise underlying causes of SEMH difficulties’. The analysis found no consensus among the HTs regarding a definition for SEMH, but identified three themes: common characteristics used to define SEMH difficulties; information seeking to identify SEMH difficulties; and how HTs recognise origins and outcomes of SEMH difficulties. The results suggest that headteachers identify behavioural ‘problems and difficulties’ as a SEN, despite this not being a category within the SEND code of practice. To improve identification and response to SEMH difficulties, it is recommended that the Department for Education revises language in statutory guidance from ‘should’ and ‘could’ to ‘must’, to enforce a legal duty on schools for prompt identification of needs.