Although educationalists, psychologists, historians, and development economists have shared an interest in levels of literacy, their research and interventions have been bedeviled by poor definitions and measurements. Early studies relied on crude indicators of absolute illiteracy, while more recent studies have tried (with little success) to identify a level of literacy necessary for everyday life, that is, “functional literacy.” Despite major international studies, our estimates and comparisons of illiteracy rates remain unreliable. The outcome is that while literacy operates as a social division similar to social class, race, gender, or physical disability, it has not been fully recognized as a form of systematic social stratification.