In 2005, the "Kappan" published a report of the authors' research on student achievement in public and private schools, based on an analysis of the 2000 National Assessment of Educational Progress (NAEP). Most people assumed that the higher average scores in private schools meant that private schools were more effective--an assumption that undergirds much of the current thinking surrounding education policies and reforms. Surprisingly, however, the data on a nationally representative sample of 30,000 students in fourth and eighth grades showed public schools to be outperforming private schools in mathematics achievement after student background factors were considered. This issue was subsequently examined with the even more comprehensive 2003 NAEP data, covering a representative sample of almost 345,000 students. Similar patterns were seen, with public schools outperforming private schools and charter schools after accounting for demographic differences in the populations they served. Later, their results were confirmed in a report published by the U.S. Department of Education. However, other researchers and some policy advocates then weighed in on this new "public/private school debate," challenging the data and methodologies used to address this question and calling for longitudinal examinations of public and private school effectiveness. In this article, the authors briefly outline the renewed debate on public and private school achievement, considering the state of current knowledge, political schisms, and the implications for current policies and proposals. They also report on a new study, not of student achievement at one point in time, but on student "gains over time" in public and private schools, using data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K). The results of this study are both intriguing and illuminating for the crucial question of the effectiveness of public and private schools. (Contains 1 table, 2 figures, and 24 notes.)