This chapter examines the legal, political, social, and cultural influence of medicine and psy-science ideas about sexual offending against children from the 1920s-1960s. Examining Australian child sexual abuse prosecutions, public advocacy, judge’s sentencing remarks, and medical literature across 40 years, we contextualise the influence of the medical models within their historical specificities. We compare lawmakers’ and state officials’ perceptions of medical explanations of child sexual offences to the views of doctors and psy-experts, analysing their often-hostile exchanges about the value of medical testimony in criminal justice settings. Conversely, we examine broader medical perspectives about the limitations of criminal law and punishment systems in preventing child sexual abuse. Overall, the paper contributes to understanding the socio-legal history of sexual offending, medicalisation, and justice institutions.