The idea of using science notebooks as a classroom assessment tool is not new. There is general agreement that science notebooks allow teachers to assess students' conceptual and procedural understanding and to provide the feedback students need for improving their performance. In this study we examined the use of science notebooks as an unobtrusive assessment tool that can also be used by individuals outside the classroom (for example, school district personnel), and as a means for obtaining information about students' learning and their opportunities to learn. More specifically, in this study students' science notebooks were used as a source of data about the (a) implementation of a curriculum's intended activities, (b) students' performance, and (c) quality of teachers' feedback. Our results indicated that: (1) Students' science notebooks can be reliably scored. Unit implementation, student performance, and teacher feedback scores were highly consistent across raters and units. (2) High and positive correlations with other performance assessment scores indicated that the student performance score can be considered as an achievement indicator. And (3) low performance scores across the two units revealed that students' communication skills and understanding were far away from the maximum score and did not improve over the course of instruction during the school year. This result may be due, in part, to the fact that no teacher feedback was found in any of the students' notebooks across the six classrooms studied. This may reflect some characteristics of the teachers' assessment practices that may require further professional development.