Over the past three decades, the mental health needs of young people who offend have become a widely recognised issue. Consequently, addressing these needs has become a priority for youth justice systems all around the world. The aim of this research was to investigate how the youth justice systems in the United States, England and Wales, and Ireland, have responded to the incarceration of young people with mental health disorders. The disproportionate number of those with mental health disorders within the youth justice system raises two questions. Firstly, what does the presence of mentally ill youth within the justice system tell us about the aims and objectives of the system? Secondly, what does the presence of mentally ill youth within the justice system mean for the operation of the system? In order to examine how these youth justice systems have responded to the incarceration of mentally ill youth, this research adopted two main methods: a comprehensive literature review and a thematic analysis of key policy documents. The literature review involved a review of secondary data from a number of sources including books, journal articles, government bodies and other relevant agencies. The thematic analysis focused on policy documents from three youth detention facilities: Oberstown Children Detention Campus in Ireland, Parc Young Offender Institution in Wales, and Santa Clara County Juvenile Hall in California. These policy documents were analysed across a three year period: 2017 to 2019. The findings of this research suggest that the progress which has been made by each jurisdiction to firstly identify and then address the mental health needs of incarcerated youth is indicative of the adoption of a welfare-based approach, thereby suggesting that punishment is not a central concern within the youth justice system. In terms of what the presence of mentally ill youth within the justice system tells us about the aims and objectives of the system, and what it means for the overall operation of the system, it is evident that the system has transformed to become a place where young people who offend can receive mental health treatment. However, society should not be reliant on the youth justice system to meet the complex needs of this vulnerable, young cohort. Instead, the mental health needs of young offenders should become the collective responsibility of society, not solely the youth justice system. Thus, this dissertation argues that the role of the youth justice system must be redefined so as to ensure that mental health services are provided outside of the system, and furthermore, that mental health is not criminalised.