How background knowledge of the content of a text affects reading comprehension and the role it plays in the diagnosis and instruction of reading comprehension skills for linguistically/culturally diverse (L/CD) students may be demonstrated through schema theory. Since a schema represents what is believed to be generally true about a class of things, events, or situations, an individual's concepts about the world can also be discussed in terms of schema theory. Studies have provided evidence that background knowledge is important in reading comprehension, especially for L/CD students. Furthermore, the linguistic and cultural experiences of an individual play a crucial role in the development of the schematic knowledge store. The closer the match between the content schema used in writing the text and the schema chosen by the reader to interpret the text, the better the level of comprehension. L/CD students can be adversely affected in their reading assessment and instruction by materials for which they lack the appropriate background knowledge. Responsible educators must be aware of the linguistic and cultural diversity of their students, and should provide appropriate materials and methods. (DF)