This chapter focuses on co-occurring substance use and mental health disorders from a social justice lens. It begins with an overview of co-occurring disorders and their significance, followed by historical and theoretical background, and concludes with practice and policy considerations. The chapter addresses significant barriers exist in their seeking appropriate treatment, including the availability and accessibility of appropriate treatment options and argues that equal concern is the role of stereotypes and stigma as barriers to services. Etiological theories of co-occurring disorders fall under four categories: common factor models, secondary mental illness models, secondary substance use disorder models, and bidirectional models. The Quadrant model underscores that treatment for individuals with comorbid disorders needs to be approached differently based on the severity of each disorder. In the field of addictions, relapse prevention has been the focus of considerable research as well as an essential part of any substance use disorder treatment plan.