The purpose of this paper is to present an overview regarding how several states use a performance or proficiency index in their determination of adequate yearly progress (AYP) under the No Child Left Behind Act of 2001 (NCLB). Typically, indexes are based on one of two weighting schemes: (1) either they weight academic performance levels--also referred to as student academic achievement levels--or (2) they weight proficiency by student enrollment--those enrolled for a full academic year--across grades in order to adjust for differing Annual Measurement Objectives (AMOs) in a school or district. In this paper, a description, chronology, and discussion of significant events in the evolution of index models since 2003 is presented together with a summary of impact data and research (to the extent that such have been identified) and an overview of the ED-approved state index models. Synopsis of Significant Events in the Evolution of Indexing Models is appended. (Contains 4 tables and 15 footnotes.)