Research shows that 7.5 million adults in Germany cannot read and write correctly. They are labeled as functional illiterates. The topic of adult illiteracy moves slowly in the focus of research. In the present paper, we give a short review over the current empirical knowledge about functional illiteracy in four areas (characteristics of the population, causes for illiteracy, literacy learning in adulthood, and adult basic education). It will be shown that despite high prevalence and social relevance, the current empirical knowledge regarding adult illiteracy is underdeveloped. Thus, we call for empirical research in four areas of investigation (population, instruction, age, and inclusion). The paper ends with a discussion of obstacles in the field that impede empirical research in adult basic education.