Complex post‐traumatic stress disorder (CPTSD) was introduced in the International Classification of Diseases (ICD) 11 in 2013 to simplify diagnosis and increase clinical utility. Given the recent ICD‐11 conceptualisation, there is no standard approach for its assessment, and a review of research is necessary. This systematic review focuses on ICD‐11 CPTSD assessment in young people aged 7 to 17 and adults aged 18 and above, examining measures, differentiating features and clinical considerations. Data from five databases are reviewed using a narrative synthesis approach and the quality of evidence is assessed and discussed. A total of 36 studies involving 5901 participants recruited from clinical settings and 1458 professionals with CPTSD assessment experience were included. Studies predominantly focused on adults, and the most used measure for assessment was the International Trauma Questionnaire. Papers focusing on differentiating features highlighted increased symptom severity, impairment and difficulties in individuals with CPTSD, compared to those with PTSD across various characteristics in both young people and adults. This review also identified the importance of a sensitive clinical approach with adaptations based on culture and age. Although gold‐standard recommendations cannot be made, this paper offers tentative clinical practice recommendations and considerations regarding ICD‐11 CPTSD assessment.