In this chapter, the medical literature on treatment for trans youth is utilised as an evaluative lens through which to consider the soundness of the law in Australia, and in England and Wales. The historic and current legal frameworks are assessed with reference to the state of medical knowledge to identify points in time where the law was and was not congruent with medicine. The medico-legal analysis focuses predominantly on an assessment of congruency of law with medicine across three key themes: (1) the nature and purpose of treatment; (2) the effects of treatment; and (3) the risks and consequences of a wrong decision being made about treatment. While historically, findings in the case law have been incongruent with the medical literature, the current legal framework is significantly more congruent than in the past in Australia. The position in the England and Wales decision analysed is broadly incongruent with respect to current medical knowledge though it is noted to no longer be authoritative.