In this article the author reviews and discusses John Hattie's book "Visible Learning". In this book Hattie presents the results of a meta-meta-analysis on the conditions of successful teaching and learning in schools. Hattie's work is based on more than 800 meta-analyses, and these meta-analyses are substantiated by more than 50 000 empirical research studies. Among the six groups of factors influencing successful learning in schools--student, home, school, teacher, curricula, teaching--the factor "teacher" seems to have the strongest effect. Hattie not only evaluates empirical research but also places the results in a theoretical context of a theory of teaching and a certain concept of teacher professionalism. The author discusses Hattie's approach to combine empirical research and conceptual work on teaching and on teachers' work. The limits of meta-analyses and future meta-meta-analyses are pointed out. (Contains 6 notes, 3 tables, and 2 figures.)