Recognizing that textbooks play a prominent role in fostering students' understanding of reasoning and proof, the authors conducted a study to examine the extent to which textbooks used in U.S. high schools provide opportunities for students to encounter proof-related reasoning and how the nature of proof-related reasoning in textbooks varies by course and publisher. Because proof has been taught primarily in high school geometry courses in the United States and there is very little research about the nature or extent of proof in other courses, the authors examined books intended for use in nongeometry courses. Specifically, they analyzed students' opportunities to learn to reason mathematically in 20 textbooks. In this article, the authors share key aspects of the framework that they used to analyze textbooks, samples of the kinds of proof-related reasoning that they found, and highlights of the quantitative results of their research. They also suggest some instructional implications, including ideas for teachers on how to use their framework to increase students' opportunities to engage with proof. (Contains 1 table.) [Bibliography of Textbook Resources is appended.]