Comprehension of expository texts is essential for middle school students to be proficient learners. Different types of knowledge such as content, text structure, and linguistic knowledge are also needed to comprehend expository text. However, it is unknown whether and how different types of knowledge influence middle school readers’ comprehension of expository texts. The purpose of this dissertation was to better understand how content, text structure, and linguistic knowledge contribute skilled and less-skilled middle school readers’ online (during) and offline (after) comprehension of simple and complex expository text. A review of the current literature on the effects of content, text structure, and linguistic knowledge on middle schoolers’ expository comprehension is presented, as well as findings from an independent study with a sample of 50 participants who completed different knowledge assessments, and a think-aloud (during) and recall task (after) reading one simple and one complex expository text. Mixed effects models were used to determine the effects of content, text structure, and linguistic knowledge, reader skill, and text complexity on middle school readers’ expository comprehension processes and products. Results indicated that content, text structure, and linguistic knowledge support the generation of expository comprehension processes and products for middle school readers in different ways. Overall, content knowledge, reader skill, and text complexity significantly impacted the types of processes generated during reading. Content, text structure, and linguistic knowledge also significantly influenced the types of products developed after reading. These findings suggest that knowledge types, reader skill, and text complexity matter for middle school readers’ expository comprehension; however, these contributions differ depending on the level of processing and development of representation and when these contributions are applied (during, after reading). Implications for theory and practice are discussed.