This special issue of the Annals of Mathematics and Artificial Intelligence contain thoroughly revised and significantly extended versions of selected papers presented at the Eleventh International Symposium on Foundations of Information and Knowledge Systems (FoIKS 2020, Dortmund, Germany, February 17–21, 2020). The call for papers solicited original contributions dealing with any foundational aspect of information and knowledge systems, including submissions that apply ideas, theories or methods from specific disciplines to information and knowledge systems. Such disciplines include discrete mathematics, logic and algebra, model theory, information theory, complexity theory, algorithmics and computation, statistics and optimization.