The effects of a voluntary summer reading intervention were assessed in a randomized field trial involving 552 students in 10 schools. In this study, fourth-grade children received 8 books to read during summer vacation, and were encouraged by their teachers to practice oral reading at home with a family member and to use comprehension strategies during independent, silent reading. Reading lessons occurred during the last month of school in June, and 8 books were mailed to students on a biweekly basis during July and August. The estimated treatment effects on a standardized test of reading achievement (Iowa Test of Basic Skills) were largest for Black students (ES = .22), Latino students (ES = .14), less fluent readers (ES = .17), and students who reported owning fewer than 50 children’s books (ES = .13). The main findings suggest that a voluntary summer reading intervention may represent a scaleable policy for improving reading achievement among lower-performing students.