Public health agencies are increasingly experimenting with quality improvement (QI) strategies designed to enhance the effectiveness and efficiency of their efforts. Does QI work in public health, and if so for whom and under what circumstances? What QI strategies work best for which types of public health process failures, and at what cost? Research underway through the Public Health Practice-Based Research Networks (PBRN) Program is examining these types of questions to build an evidence base for public health QI.