Does reading practice make perfect? Or is reading achievement related to the quality of practice as well as the quantity? To answer these questions, data on 45,670 students in grades 1-12 who read over 3 million books were analyzed. Measures largely of quantity (engaged reading volume) and purely of quality (success in reading comprehension) showed a positive relationship with achievement gain at all levels of achievement. However, both high quantity and high quality in combination were necessary for high achievement gains, especially for older students. Both were weakly associated with student initial reading achievement, but more strongly associated with the classroom in which the student was enrolled, possibly suggesting the properties of teacher intervention in guiding independent reading were important. Implications for theory-building, research and practice are explored.