ABSTRACT Support is requested for a Keystone Symposia conference entitled Immunity in Early Life: From the Maternal to the Microbial Environment, organized by Drs. Chris Wilson, Tobias Kollmann and Claire-Anne Siegrist. The conference will be held in Keystone, Colorado from June 7-11, 2020. The immune system is crucial for host defense from infection and malignancy. It impacts nearly all of host physiology and, when misdirected, causes allergic and autoimmune disease. Recently, it has become increasingly clear that early life is a key period during which host genetics, developmental programs, and the environment (from infection to vaccination to microbiome etc.) shape the immune system. This imprint is then carried forward through childhood into adult life. Yet, this early life period represents not only a window of vulnerability but of opportunity, where targeted interventions could provide lifelong benefits. This Keystone Symposium conference examines early life immunity, bringing together experts from a sundry range of disciplines. This diverse group will present a tapestry of extant knowledge and insights, provide a more comprehensive understanding of immune ontogeny, and illuminate areas of uncertainty where further study is most needed. Conference participants will leave with an understanding of the role of microbiome in early life immune ontogeny. In addition, discussion on the precise mechanistic role of early life immunity in health versus disease will be included in sessions. Finally, given that crucial events in immune ontogeny may be influenced by prenatal events, this conference will be held jointly with Maternal-Fetal Crosstalk, which emphasizes the strong overlap and interdependence of fetal and neonatal with maternal immunity. There will be joint sessions which will provide the unique opportunity to cross-fertilize pre- and postnatal immunity.