Background: The VA Grant and Per Diem case management (GPD-CM) program provides six months of case management for homeless-experienced Veterans (HEVs) undergoing housing transitions. At present, no specific case management paradigm is required in the GPD-CM program, resulting in practice variation across sites. National implementation of Critical Time Intervention (CTI)—an evidence-based, structured, and time- limited case management program for HEVs experiencing housing transitions—would standardize and improve case management in the GPD-CM program. Implementing CTI in routine care settings requires balancing the need for CTI fidelity with adaptations to fit the diverse programs serving this population. Moreover, some organizational contexts may require more intense and tailored supports to implement CTI. Objectives: We propose to implement CTI at 32 GPD-CM sites. To implement and sustain CTI across these sites, we will use the Replicating Effective Programs (REP) implementation bundle to enable sites to achieve fidelity to CTI’s core components, while accommodating adaptations to fit the diversity of GPD-CM settings and contexts. We will cluster randomize half the sites to also receive 9 months of external facilitation (“enhanced REP”), an established process of providing tailored support for providers and leaders in the ir efforts to adopt and incorporate EBPs into their routine care processes. Our Specific Aims are to: 1) Use REP and enhanced REP to support the implementation and sustainment of CTI in 32 GPD -CM sites; 2) Compare, in a type 3 hybrid implementation-effectiveness trial, the impacts of REP vs. enhanced REP on CTI fidelity and sustainment, quality metrics (focused on housing stability and hospitalization rates), and costs and return-on- investment; and 3) Generate two key products for program partners—a business case analysis and an implementation playbook —to support continued spread and sustainment of CTI in the GPD -CM program. Methods: We will use a CTI training and technical assistance package developed and refined in our QUERI- VISN Partnered Implementation Initiative (PII) Start-up to implement CTI in 32 GPD-CM sites. We will use REP to support CTI implementation at all 32 sites. Half of these sites will also receive 9 months of external facilitation (EF, as part of enhanced REP), building on our EF experiences and materials developed in the PII Start-up. To compare the impacts of REP vs. enhanced REP, we will conduct a cluster randomized type 3 hybrid trial. We will use a rollout design in which randomization occurs at two levels: when impleme ntation begins (three cohorts) and the implementation strategy (REP vs. enhanced REP). We will use permuted block randomization to balance key site characteristics among sites receiving each of these strategies across cohorts. We will use mixed methods to assess the impacts of REP vs. enhanced REP. As fidelity to CTI influences its effectiveness, fidelity to CTI is our primary outco...