This paper presents a theory of instructional text design that takes into account the process of reading comprehension and the variables in both the reader and the text which can affect that process. Three areas of design concern--legibility and readability, visual illustration, and structure and organization--are discussed. Each area design concern is considered as contributing either to the "interactivity" of the process or to the structure and organization aspect of acquired knowledge in memory. A unifying theory incorporating all three areas of concern is then presented which is based on findings of prior research that: (1) reading is an interactive process between reader and text; (2) the process is contributed to by characteristics of both the text and the reader; and (3) knowledge in memory is highly structured and organized. It is argued that, given these factors, design elements of the text can be manipulated to ensure that existing knowledge in the reader can be activated so that new knowledge presented in the text in a structured and organized way and appropriately highlighted through verbal and typographic cuing supported, as required, by visual illustration and organization, can be assimilated in a manner facilitative of reading comprehension. One diagram is provided. (28 references) (EW)