Onsite wastewater treatment systems (OWTS; widely known as "septic systems") serve approximately 25% of the U.S. population. In rural and unsewered watersheds, they are the only means to treat wastewater. As federal subsidies for funding centralized municipal treatment have ended, new development in rural areas is almost exclusively dependent on OWTS. Typically these systems serve individual rural residences and small farmsteads, but may also treat flows from small community cluster treatment systems. New challenges necessitate advances in our understanding, design and management of OWTS that warrant a multi-state effort of Land Grant scientists. Of particular note are three drivers of change: (1) OWTS are increasingly in use for decades and are used for all types of rural establishments, raising concerns for long-term hydraulic and treatment performance, (2) increasing demands by public and environmental health professionals that OWTS achieve high and reliable levels of N, P, emerging chemicals and pathogen removal rates - well beyond those associated with conventional technologies, and (3) challenges presented by anticipated changes in climate, such as rising water tables or severe drought, which argue for new approaches to site selection, design and water reuse. The enormous variation in soils, geology, climate and hydrology that affect the proper function of OWTS across the nation requires multi-regional collaboration to ensure standardization of state-of-the-science and engineering approaches to assessment and protocols. We have assembled a diverse working team of university scientists, along with industry, regulatory agency personnel and private sector wastewater practitioner partners from around the nation to begin to address the challenges faced by OWTS from the design, assessment and management perspectives in the context of climate change.