Finding the right fit between students' reading ability and textbooks is very important for comprehension. Readability studies aim to analyse texts to find the right fit between students and texts. In this literature review, readability studies are classified under quantitative, qualitative and combined quantitative-qualitative readability approaches. The quantitative approach includes readability formulas, cloze test, and checklists and scales. The qualitative approach consists of leveling and checklists. The combined qualitative and quantitative readability approach is new in the field. In this approach, readability formulas can be used together with benchmark passages and checklists. The literature shows that readability formulas rely heavily on surface features of a text, and gives a rough estimate of the text readability. The qualitative approach focuses on the quality of writing style, and is criticised as being too subjective. The paper concludes by evaluating the implications of readability studies for Turkey.