The authors tested whether students can be taught to better understand conventional representations in diagrams, photographs, and other visual representations in science textbooks. The authors developed a teacher-delivered, workbook-and-discussion-based classroom instructional method called Conventions of Diagrams (COD). The authors trained 1 experienced teacher to deliver COD to two 10th-grade biology classes ("n" = 31) and compared gains in diagram comprehension from COD to those from a business-as-usual control condition ("n" = 30) in 2 classrooms taught by the same teacher. Students in the COD condition showed statistically significantly greater growth in comprehension of literal and inferential biology diagrams. The control condition in some cases advantaged high-spatial, high-knowledge students, whereas the COD condition for the most part did not. Entries in the COD workbooks were analyzed for amount of student effort. Students with a combination of low pretest biology knowledge and low effort showed much lower gains from pretest to posttest on the inferential biology diagrams measure than did other students in the COD condition.