Background: Many Veterans experience impairing symptoms of anxiety and seek treatment in primary care, yet anxiety is highly undertreated in this setting. Primary Care-Mental Health Integration (PCMHI), in which mental health clinicians embedded in primary care provide brief treatment, affords an opportunity to address this treatment gap. However, behavioral interventions that are compatible with the brief PCMHI treatment format and able to accommodate a wide range of anxiety presentations are needed. Accordingly, Veterans Anxiety Skills Training (VAST) was designed to be evidence-based, transdiagnostic, feasible for PCMHI, and Veteran-centered. VAST comprises empirically-supported cognitive-behavioral therapy (CBT) techniques adapted into a manualized modular intervention specifically designed for delivery in PCMHI. As VAST is being piloted at a single site with study therapists, the next scientific step is a multi-site randomized controlled trial (RCT) to evaluate its effectiveness using PCMHI providers in routine care. Significance/Impact: This study addresses Veteran care priorities of mental health and suicide prevention, MISSION Act priorities of increasing access to care and improving patient satisfaction with Veterans Health Administration (VHA) care, and Health Services Research and Development (HSR&D) focus areas of quality, effectiveness, and efficiency as well as implementation science research methods. Improving anxiety treatment in PCMHI will address a major gap in VHA treatment options, given the high prevalence of anxiety in primary care, tendency for patients with anxiety to seek treatment in primary care rather than specialty mental health settings, and absence of Evidence-Based Psychotherapy (EBP) protocols addressing non-trauma related anxiety. Innovation: Modular (vs. standard) intervention designs offer advantages in efficiency, patient and provider satisfaction, efficacy, effectiveness, implementation, and sustainability. Other innovative aspects include the transdiagnostic approach and examination of fidelity to inform future implementation. Specific Aims: The specific aims are to: (1) Compare patient clinical outcomes for VAST vs. PCMHI usual care between baseline and 16 weeks (post) in a multi-site RCT in which PCMHI providers deliver VAST, and examine whether treatment gains are more likely to be maintained for VAST vs. PCMHI usual care at 28 weeks (follow-up); (2) For participants assigned to receive VAST, explore patient-level predictors of (a) early (8 weeks) and (b) overall (16 weeks) treatment response; and (3) Conduct a mixed-methods process evaluation of VAST implementation to examine rates of, barriers to, and facilitators of achieving and sustaining high-fidelity intervention delivery. Methodology: In this multi-site, hybrid type I effectiveness-implementation RCT, 178 adult Veterans with elevated anxiety symptoms will be recruited from primary care at two VHA sites. PCMHI providers will be randomized to delive...