This thesis examines the development of child psychiatric theory and practice in Denmark and England from 1900 to c. 1960. Previous studies of the development of child psychiatry have been few and limited mainly to theoretical developments, specific diagnoses, or institutional developments from the 1950s and onwards. This thesis examines the period leading up to the official constitution of child psychiatry as a medical specialty in the 1950s, focusing on how the medical understanding of mental illness in children developed before, during and immediately after the Second World War. This thesis is structured around four different themes. First, the analysis of influential, medical articles and textbooks published in the period 1900-1960 demonstrates how child psychiatric theories on aetiology shifted markedly around the late 1930s and 1940s, but also reveal that child psychiatric taxonomies remained vague. Second, the analysis of conference publications related to the first, second, third and sixth international child psychiatric conferences reveal how the Second World War played a crucial role in the development of child psychiatry as a fundamental building block of the welfare state. Third, exploring the concerns of child psychiatrists regarding ‘war-damaged children‘, the English case of the Windermere children is investigated and compared to treatment of traumatized children in Denmark. Fourth, child psychiatric practices at the Copenhagen University Clinic and the Tavistock Clinic (London) are examined via clinical material and annual reports revealing differences in the use of psychoanalytic concepts. Compared to the significant theoretical developments of the medical articles and textbooks as well as the international conferences, this thesis demonstrates that changes in the psychiatric practices were less conspicuous and that child psychiatric taxonomies remained vague. Generally, the thesis provides a nuanced understanding of how child psychiatry changed both scientific and socio-political status in the aftermath of the Second World War.