Objectives. We sought to identify factors believed to facilitate or hinder evidence-based practice (EBP) implementation in public mental health service systems as a step in developing theory to be tested in future studies. Methods. Focusing across levels of an entire large public sector mental health service system for youths, we engaged participants from 6 stakeholder groups: county officials, agency directors, program managers, clinical staff, administrative staff, and consumers. Results. Participants generated 105 unique statements identifying implementation barriers and facilitators. Participants rated each statement on importance and changeability (i.e., the degree to which each barrier or facilitator is considered changeable). Data analyses distilled statements into 14 factors or dimensions. Descriptive analyses suggest that perceptions of importance and changeability varied across stakeholder groups. Conclusions. Implementation of EBP is a complex process. Cross-system–level approaches are needed to bring divergent and convergent perspectives to light. Examples include agency and program directors facilitating EBP implementation by supporting staff, actively sharing information with policymakers and administrators about EBP effectiveness and fit with clients' needs and preferences, and helping clinicians to present and deliver EBPs and address consumer concerns.