The topic of this study is girls and boys in the field of child welfare. The aim has been to trace and describe conceptions of gender, delinquency and social problems in child welfare from the end of the nineteenth century until the middle of the twentieth century. The sources of data in the study consist of legislative documents, journals and other historical literature representing the professional discourse. Two analytical approaches have been used. First, the sources were employed for descriptions of legislation and the legislative processes in child welfare. Second, the material was analyzed with a discursive approach to elucidate conceptions of gender and their importance in the legislative process and thus in the construction of social problems. Three main periods with different currants of ideas have been identified. In the first period moralism dominated. Thoughts about social problems were based in normative assumptions and scientific influences were rare. Child welfare legislation was deeply influenced by ideas of social control. Young people were to be controlled and disciplined especially through work: Girls through household work and care and boys through paid work. In the second period hygienism, ideas based in hygiene discourses, were the main trend. Genetically based arguments together with discussions about morality and poverty were used to explain social problems, and scientific methods were to be used to discipline and control young people. Proposals for measures take against social problems corresponded to two main lines, a hygienic-medical line and a social pedagogic line. The hygienic-medical line had a considerable influence on actions taken to prevent and deal with social problems. One example was the compulsory care and sterilization of certain young women to prevent them from reproducing. The social pedagogic line comprised parents’ education, the role of the family and sexual education. An increased use of psychological explanations for experiences and behaviour among individuals and groups was seen in the 1930’s; this marked the beginning of the third period, psychologism. During the period of psychologism, science, mainly represented by psychology and psychiatry, gradually achieved a greater impact in those processes where young people were categorized as social problems. If in the previous period external discipline was the means of control, in this period internalized self-discipline was to be the means of adjustment. The study showed that conceptions of girls and boys in child welfare were gendered throughout the first half of the twentieth century. This bias has had an impact on how girls and boys were treated in child welfare services. Discussions about social problems concerning girls were often about sexuality and sexual actions, and discussions concerning boys were about violence and criminality. Social problems among girls were seen above all as moral transgressions and social problems among boys as juridical transgressions. These differing conceptions are of great importance when considering how girls and boys were judged and treated according to the child welfare Acts in existence during the first part of the twentieth century.