Engineering notebooks are a pervasive practice across high school, college, and professional contexts. Within this consistency, there are two basic forms: process-based notebooks serve as a complete record of the engineer's work and are used by the authors to support their endeavors, while product-based notebooks represent a record of final form projects to be evaluated by outsiders. Given this breadth of purposes, expected content and primary audiences, this paper explores students' and teacher's use and perceptions of the utility of engineering notebooks. In this study, we interviewed students and teachers, and examined the student notebooks, from three different high-school engineering classes. Grounded-theory analyses of this data reveals that all classes used process-based notebooks and they all struggled with supporting student reflection, a common teacher goal. In addition, in each class, the notebooks emphasized a different phase of the design process and students similarly reported the utility of notebooks for supporting that phase. The key instructional implication of this work is that facilitating students' meaningful and thorough use of their notebooks requires creating situations in which maintaining notebooks helps them complete their project work. The paper provides examples, pulled from the study data, of how to enact this strategy.