In Sweden, units managed by nurses specialised in counselling and telephone triage, have been developed within the Child and Adolescent Mental Health services (CAMHS). This study has a qualitative design and illuminates the nurses' perceptions of what helps or hinders their assessments and telephone triage. The Enhanced Critical Incident Technique was utilised, eight nurses were interviewed in depth, to identify factors influencing triage. The study is the first to provide a comprehensive description of helpful and hindering factors while performing telephone triage. It illuminates telephone triage in Swedish CAMHS settings and provides insights how to enhance and implement this practice.