This article advances the first attempt to collect and examine empirical data on young adult (YA) spaces in public libraries from institutions across the United States by surveying current practices in new and renovated buildings. Analysis of an online survey of 257 library and information science (LIS) professionals produced an innovative Youth Participation Index (YPI) used to document the relative intensities of youth involvement in the design and execution of YA spaces. Libraries claiming higher levels of youth participation reported significant quality service improvements across a wide range of outcomes. However, after several decades of advocating for youth involvement in the delivery of library services from many LIS sources, only a minority of the libraries surveyed reported high YPI scores, and the specific mechanisms for enacting youth involvement require further investigation.