Illiterates represent a significant proportion of the world's population. Written language not only plays a role in mediating cognition, but also extends our knowledge of the world. Two major reasons for illiteracy can be distinguished, social (e.g., absence of schools), and personal (e.g., learning difficulties). Without written language, our knowledge of the external world is partially limited by immediate sensory information and concrete environmental conditions. Literacy is significantly associated with virtually all neuropsychological measures, even though the correlation between education and neuropsychological test scores depends on the specific test. The impact of literacy is reflected in different spheres of cognitive functioning. Learning to read reinforces and modifies certain fundamental abilities, such as verbal and visual memory, phonological awareness, and visuospatial and visuomotor skills. Functional imaging studies are now demonstrating that literacy and education influence the pathways used by the brain for problem-solving. The existence of partially specific neuronal networks as a probable consequence of the literacy level supports the hypothesis that education impacts not only the individual's day-to-day strategies, but also the brain networks. A review of the issues related to dementia in illiterates is presented, emphasizing that the association between the education level and age-related cognitive changes and education remains controversial. The analysis of the impact of illiteracy on neuropsychological test performance represents a crucial approach to understanding human cognition and its brain organization under normal and abnormal conditions.