The Pathways to Enable Secure Open-Source Ecosystems (PESOSE) program supports the translation of open-source science and engineering-focused research products into safe and sustainable ecosystems that address national and societal challenges. Open-source tools such as software, hardware, machine learning models, languages, and data platforms are designed to be shared as they are publicly-accessible and modifiable. These tools spark innovation in critical fields as varied as artificial intelligence (AI) and cloud computing, banking, healthcare, research, education, next-gen manufacturing, mobility, and National security (including cybersecurity). PESOSE supports the creation of managing organizations for these ecosystems, ensuring strong governance, distributed development, and broad user communities across academia, industry, and government. PESOSE also supports enhancements to the safety, security, and privacy of Open-Source Ecosystems (OSE) by addressing significant vulnerabilities, both technical and socio-technical, to improve the resistance of the ecosystem against threats. This solicitation seeks three types of proposals, allowing teams to propose specific activities to:1) scope and planthe establishment of an OSE, 2)establishand expand a sustainable OSE based on a robust, promising open-source product that meets an emergent societal or national need, and 3) improve the safety, security, and privacy of an existing OSE and its products.