Background and Purpose: Depression affects millions of adolescents in the United States each year. This population may benefit from targeted preventive interventions. We sought to understand the internal factors that affect the ability of healthcare organizations to implement an intervention that involves mental health screening and depression prevention treatment of at-risk adolescents in primary care settings. Methods: From November 2011 to July 2016 we conducted a study of the implementation of a multisite (N=30) phase 3 randomized clinical trial of an Internet-based depression prevention intervention program (CATCH-IT). We describe the prevalence of internal barriers on the screening and enrollment process by reporting REACH (the proportion of target audience exposed to the intervention). Results: A total of 369 adolescents were randomized into the intervention or control program. Mean REACH values for the study clinics were 0.216 for screening and 0.181 for enrollment to CATCH-IT. Mean REACH enrollment lost due to internal barriers was 0.233. This translated to 4,691 adolescents lost at screening and 2,443 adolescents lost at enrollment due to internal barriers. Conclusion: We propose a model of the implementation process that emphasizes the importance of positive relational work that assists in overcoming internal barriers to REACH. We also provide implications for policy and practice.