In recent years, John Hattie's book Visible Learning (2009) has greatly influenced educational practitioners and policymakers. The visible learning approach has been deemed "the Holy Grail of teaching" (Mansell, 2008), and Hattie has been called the "messiah" of educational research (Evans, 2012). In this article, we outline some of the significant methodological problems embedded in Hattie's work and relate them to his theoretical