Background: More than thirty years after the beginning of the human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic, HIV continues to be a problem among adolescents and young adults (AYA).. The benefits of combination antiretroviral therapy (cART), including lower morbidity and improved survival, are realized through timely uptake of treatment, virological suppression, and retention in care; however, compared with their adult counterparts, AYA living with HIV tend to have poorer treatment and clinical outcomes. In the current context of UNAIDS’ ambitious 90-90-90 campaign, there is a push to expand cART to all those in need in order to reduce morbidity/mortality and to curb transmission of HIV. We will not achieve the 90-90-90 goals without addressing HIV treatment and outcomes among AYA; however this population remains under-researched; to date there is very little research describing AYA living with HIV in Canada. The overall aim of this dissertation is to examine key cART treatment outcomes among Canadian AYA (ages 18-29 years) living with HIV and compare outcomes with those of older adults (30 years and older). Methods: The quantitative studies in this dissertation utilize clinical and laboratory data from the Canadian Observational Cohort Collaboration (CANOC), Canada’s largest HIV cohort study, which includes data from the year 2000 onward on a total of 10,044 people living in three of Canada’s largest provinces—British Columbia, Ontario, and Quebec. All participants in CANOC are HIV-positive and were cART-naive prior to initiating antiretroviral treatment on or after January 1, 2000. Findings: There are 1168 (13.7%) AYA (ages 18-29 years) in CANOC. Significant differences in treatment outcomes were found between AYA and older adults. AYA were more likely than older adults to initiate cART before their CD4 counts were <200 cells/mm3 and/or they had an AIDS-defining illness (ADI) (51.7% vs 40.2% p<0.001). When looking at virological suppression, fewer AYA experienced virological suppression than older adults (86% vs. 91%, p<0.001) and of these, only 73% (compared with 80% of older adults) suppressed within the first year of cART inititation (p<0.001). Additionally, a greater proportion of AYA who achieved virological suppression experienced viral rebound than older adults (26% vs. 22%, p=0.009). Discussion: When comparing AYA with older adults, AYA are more likely to initiate treatment when recommended but once on treatment, they are less likely to virologically suppress and remain suppressed. The importance of supporting AYA to achieve optimal health is a long-term investment with benefits over the life course. In order to meet ambitious public health goals such as those that the UNIADS 90-90-90 campaign has set, AYA will require tailored health care services and programming to assist them to access and remain in care.