PROJECT SUMMARY/ABSTRACT The prevalence of mental disorder is three times greater in humanitarian settings relative to global averages. Challenges to implementing mental health services in complex emergencies amplify health disparities for hard- to-reach populations. Migrants transiting to a destination country are among those hardest to reach and retain in care given their mobility and peripheral connections with established health systems. Innovative strategies are needed to improve access to mental health services for migrants in transit. The goal of this K01 Mentored Research Scientist Development Award is to prepare the candidate for an independent research career dedicated to improving access to evidence-based mental health services for the hardest-to-reach populations in humanitarian emergencies. Through this K01 award, the candidate will leverage the training resources available through Columbia University and her mentorship team to acquire the skills needed in systems science and human-centered design to develop and test strategies for improving reach and retention of migrants in transit in scalable psychological interventions. The goal of this K01 research is to optimize implementation of Problem Management Plus (PM+), a scalable psychological intervention, to improve reach and retention of Venezuelan migrants transiting through Colombia. This research builds on an ongoing research partnership between Columbia University and HIAS, a humanitarian non-governmental organization in Colombia. The specific aims are: 1) To describe the individual-, community-, and systems-level relationships that influence reach and retention in mental health services among Venezuelan migrants in transit in Colombia using group model building; 2) To design and optimize an implementation plan to improve reach and retention in PM+ among Venezuelan migrants in transit in Colombia through human-centered design; and 3) To compare reach and retention in PM+ using the locally designed implementation plan relative to PM+ implementation-as-usual among Venezuelan migrants in transit in Colombia with a comparative interrupted time series analysis. This research is an integral component of a rigorous training and career development plan involving intensive training and mentorship in systems science, human-centered design, adapting implementation strategies, and understanding structural, cultural, and social determinants of access to care among migrants in transit in Latin America. This study will be the first to optimize implementation of mental health services for migrants in transit and will provide critical preliminary data to inform an R01 application to test promising implementation approaches at scale. This research aligns with NIMH’s strategic objective to develop innovative service delivery models to improve mental health in diverse communities and populations, as well as the priorities of the NIMH Global Mental Health and Human Mobility Research Program.