The principles of readability are in every style manual. Readability formulas are in every writing aid. What is missing is the research and theory on which they stand. This short review of readability research spans 100 years. The first part covers the history of adult literacy studies in the U.S., establishing the stratified nature of the adult reading public. The second part covers the classic readability studies (prior to 1950) showing the development of vocabulary frequency lists and the first readability formulas. The third part covers the new readability studies of the last 50 years, showing the features of texts and readers that govern reading ease. The review attempts to resolve the controversies surrounding the readability formulas.