Background: Assessment of mental health in schools has garnered increased interest in recent years. Children spend a large proportion of their waking hours in schools. Teachers can act as gatekeepers by playing a key role in identifying children with mental health difficulties in the classroom and making the necessary onward referrals to external services. The prevalence of mental health difficulties, their impact on schooling (and beyond) and the importance of early intervention means that it is incumbent on schools to identify and support potentially affected children. Aims: Previous reviews focused on mental health interventions in schools; however, this review focuses on the assessment of mental health in schools and on teachers' perceptions of this, as such a review is still lacking. Therefore, the study fills a gap in the existing literature while also providing new, highly relevant evidence that may inform policy making in this area. Composition of studies included in this review: This review included 19 studies. Five studied teachers exclusively at primary/elementary level, and seven focused on secondary level, while six included both primary and secondary teachers. Three studies employed mixed methods, ten were primarily qualitative studies, and five were primarily quantitative. Methods: Bronfenbrenner's (The ecology of human development: Experiments by nature and design, Harvard University Press, 1979) framework, adapted by Harvest (How can EPs best support secondary school staff to work effectively with children and young people who experience social, emotional and mental health difficulties? 2018), which includes the mature version of the theory (Tudge et al., 2009, J. Fam. Theory Rev., 1, 198), was used to analyse the literature. Results: Results found that lack of training in assessment of mental health and 'role conflict' were key barriers; some teachers attributed this to their lack of knowledge, skills and confidence in the area. Conclusion: Implications for practice and research are discussed in relation to the importance of sustained training both pre-service and in-service.