Between 1998 and 2014, the Promising Practices Network (PPN) on Children, Families and Communities website (www.promisingpractices.net) provided information on programs and practices that credible research indicated are effective in improving outcomes for children, youth, and families. This document was originally published as part of PPN's Issue Brief series, and it is intended to help PPN visitors understand some of the variation in how the field of child and family services defines "best available research evidence." The authors describe the history of evidence-based practices and review varying definitions of them, comparing the PPN criteria with those from other organizations.