JustIn: An Open Source Software Platform and Community to Accelerate Adaptive Mobile Intervention Research - Abstract: Meaningful impact on the prevalence and incidence of substance use disorders (SUD) and HIV requires an array of evidence-based interventions that address individuals’ changing needs, circumstances, and strengths. Digital adaptive interventions (DAIs) that leverage advances in digital technologies such as mobile devices, wearable sensors, and artificial intelligence have shown great promise in improving outcomes for managing SUDs and preventing HIV, as well as for improving outcomes in other health domains. However, such interventions remain difficult and resource- intensive to build, and existing approaches to building them fail to support dissemination of best practices in both methodology and building the evidence base. In this project we will build and make available JustIn: a sustainable open source software (OSS) platform and community for advancing research in adaptive interventions. Within the scope of this project we will develop and release an initial version of JustIn that supports the majority of the functionality required to create existing state of the art adaptive interventions. Along with the initial release we will provide comprehensive documentation, tutorials, and working sample application code to bootstrap the adoption of JustIn for future intervention development. Following best practices for creating sustainable OSS communities, we will develop guidelines and procedures for integrating code contributions from the wider research community and implement governance structures that facilitate community input on features and priorities for future versions of JustIn. JustIn will democratize research to optimize digital adaptive interventions in SUD/HIV and other domains of health, allowing investigators to keep pace with emerging technologies and leverage them to produce radically effective and resource- efficient interventions. The growth in systematic development of novel digital adaptive interventions has the potential to reduce the burden of SUD/HIV and other chronic disorders.