The purpose of this paper is to make classroom teachers on all educational levels aware of the limitations of readability formulas and the factors affecting readability that are not accounted for by the readability formulas. delineated in discussions of readability formula assumptions, problems, and limitations. Some of the specific factors that are discussed include the reader's prior knowledge of the subject matter, format, organization, text structure, content, style, flow of information, lack of conflict structure, idea and inference density, grammatical and psycholinguistic factors, and reader characteristics. Based on the discussion of these factors, the conclusions section of the paper considers the need for different readability formulas for different users--simpler formulas for classroom teachers doing gross screenings of reading materials and more sophisticated formulas for textbook publishers and textbook adoption committees--and the need for teachers to use readability formulas with understanding and caution. (RL)