In the United States (US), substance use, misuse, and substance use disorders are significant public health problems. Additionally, the difference between needing substance use treatment (SUT) and receiving treatment is becoming more pronounced. Therefore, the aims of this dissertation include, 1) to synthesize evidence of barriers and facilitators to integrating SUT into mainstream health care (MHC) after the Affordable Care Act was nationally implemented in 2014, 2) to examine the rate of treatment completion and dropout, along with its associated factors, among youth who received Medication-Assisted Opioid Therapy (MAT), 3) to determine the relationship between prior treatment episodes and length of stay (LOS) among adults in residential rehabilitation facilities (RRF) and explore other predictors of LOS and, 4) to determine whether experiencing multiple treatment episodes and being in certain age groups was associated with longer wait period to enter SUT. The literature synthesis involved data from five databases and was informed by PRISMA. Quantitative analysis (aims 2-4) included log-binomial, Poisson, and logistic regression models using the Treatment Episode Data Set-Admissions and Discharges. Andersen’s Behavioral Model for Health Services was the conceptual framework used to inform aims 2-4. For aim 1, several patient, provider, and program/system-level barriers and facilitators were identified, which highlights the need to use a comprehensive approach to improve SUT adoption in MHC. For aim 2, among youth who received MAT, 43.9% completed treatment and 56.1% dropped out. Factors positively associated with treatment completion included MAT use, males, self-help group participation, admission to detoxification and residential/rehabilitation settings, and being in the Midwest/Western US; while minority races (excluding Blacks/African Americans) and being in the South resulted in lower likelihood. For aim 3, the average LOS in RRF was 16.4 days, and having ≥ 1 prior SUT episode (PSUTE) was associated with slightly higher LOS. Some predictors associated with lower LOS included age (18-34 years), males, being employed, private insurance, and being in the Northeast. For aim 4, a longer wait period was identified among clients 25-49 years and those with ≥5 PSUTEs. These factors should be considered to improve SUT use, and future studies should corroborate these findings.