The impact of class time lengths on student achievement is a complex issue with multiple extraneous factors and without definitive answers. A major theme across many of the studies reviewed is that the amount of instructional time is not so important as how that time is spent. Key points include: (1) The commonly held conception that students in other countries outperform U.S. students because they spend more time in school is not supported by research; (2) Neither increased days/year nor hours/day have been shown to increase student learning significantly except in special circumstances; and (3) Studies on block scheduling have reported mixed results.