Background: Persons who inject drugs (PWID) are at increased risk for infectious diseases and overdose, and need access to primary care and substance use disorder (SUD) services. Yet, PWID historically have avoided and delayed seeking care, in part due to lacking insurance coverage for services they need and an availability of these services at local clinics. The Affordable Care Act (ACA) made insurance mandatory and included SUD services as an essential health benefit. Objectives: This dissertation research sought to examine the impact of the implementation of the ACA on the use of SUD treatment (Chapter 2) and primary care services (Chapter 3) among PWID in San Diego. It also sought to understand the changes in offering of SUD services at community and free primary care clinics after this health reform was implemented (Chapter 4). Methods: This dissertation is comprised of three papers assessing different aspects of health service use among PWID in San Diego County. Papers one and two (Chapters 2 and 3) used data from a longitudinal cohort study of 576 PWID in San Diego (STAHR 2 Study) to determine the impact of the ACA on the use of SUD treatment and the use of primary care services, respectively. Paper three (Chapter 3) used facility-level data on community and free primary care clinics in San Diego County from the California Office of Statewide Health Planning and Development to understand changes in offering of SUD services at these clinics after the ACA compared to before. Results: For paper one (Chapter 2), there was an increase (12%) in use of SUD treatment after the ACA compared to before, with a similar increase (10%) in insurance coverage. The ACA was associated with greater use of SUD treatment among PWID and the strength of this association was not impacted by insurance coverage. For paper two (Chapter 3), there was an increase (4%) in primary care use after the ACA compared to before, with a much larger increase (24%) in those who had insurance coverage. The ACA was associated with greater use of primary care services among PWID and the strength of the association was not entirely explained by an increase in insurance coverage. Paper three (Chapter 4) showed an decrease of 7% in the number of community and primary care clinics that offered SUD services after the ACA compared to before, but an increase in the proportion of patients covered by Medi-Cal and had contact with a substance abuse counselor. Conclusion: These findings suggest that the ACA may have been an effective health policy for increasing access and use of essential health services among PWID in San Diego, and for bolstering SUD services in community and free primary care clinics.