A study assessed the relationship between formal main idea statements in text and substantive material that students need to understand the key concepts in textbooks. The study addressed two questions: (1) What percentage of textbook paragraphs contains material important for understanding major textbook concepts? and (2) How important are formal main ideas for a substantive understanding of textbook material? Chapters from three textbooks representing mainstream sociology texts were examined. Areas covered by the chapters include culture, the family, and social inequality, topics treated somewhat uniformly in most mainstream textbooks. To distinguish between "substantive" and "useful" material, the number of paragraphs in each chapter containing substantive material was compared with the total number of paragraphs in each chapter. Location of paragraphs without substantive material and the kind of material contained therein were also noted. Results of analysis indicated that large numbers of paragraphs contained no substantive ideas; the importance of finding main ideas, which were significant 79% to 91% of the time; and the need to study supporting details in order to gain understanding of important material. In most of the paragraphs containing important information, one or more supporting details were needed to understand what was being said, while the formal main idea was sometimes too general and contained little of value for understanding the subject matter. (HTH)