PROJECT SUMMARY Sustainment of evidence-based interventions (EBIs) is a priority area for the field of implementation science. However, there are notable gaps in the field’s understanding of how implementation strategies prepare practitioners to sustain EBIs. Mothers and Babies (MB) is an EBI focused on prevention of perinatal depression that is widely implemented across home visiting (HV) programs that serve pregnant and parenting individuals. Specific implementation strategies have been developed and are being used to support MB sustainment in HV programs, with state HV networks in New Mexico and Rhode Island transitioning from an initial implementation (0-24 months) to sustainment (24-36 months) phase during the project period. The proposed R21 project will leverage the research team’s partnership with these networks and has two specific aims. First, we will determine how existing implementation strategies prepare HV programs to sustain MB. We will collect administrative data to assess the extent to which MB is sustained at 24 months, inclusive of provider- and client-level sustainment outcomes. We will also elicit perspectives from New Mexico and Rhode Island HV mangers, staff, and clients on the extent to which sustainment predictors are associated with fidelity of implementation strategies and the perceive usefulness of implementation strategies in promoting MB sustainment. Analyses will also be conducted examining the relationship between sustainment predictors and sustainment outcomes. Qualitative data will also be collected to assess how implementation strategies influence sustainment predictors and sustainment outcomes, as well as multi-level contextual factors that affect how implementation strategies enhance sustainment predictors and sustainment outcomes. Second, we will develop and pilot test an implementation strategy “bundle” to support MB sustainment in HV. We will use a human-centered design approach and work with HV managers and staff to finalize an implementation strategy bundle that supports MB sustainment in HV, and pilot test the new implementation strategy bundle with two new state HV networks to examine the feasibility, acceptability, and appropriateness of each strategy. These activities will prepare us to conduct a R01 hybrid-effectiveness Type 2 trial testing the new implementation strategy bundle as well as sustained MB implementation and effectiveness among HV programs receiving the strategy bundle. Public health significance and innovation of this project is substantial. Perinatal depression is the most common health complication associated with pregnancy, affecting one in five women. HV programs provide services to pregnant and parenting individuals across all 50 states. By developing a sustainment model for integrating MB into HV, this R21 will facilitate the sustained delivery of MB, which is consistent with NIMH’s strategic objective to scale-up evidence-based practices and goal of PAR-22-109 to identify, develo...