This book contains selected papers from IWIFSGN-2023, the Twenty First International Workshop on Intuitionistic Fuzzy Sets and Generalized Nets, held on October 20, 2023, in Warsaw. This is one of premiere conferences on fuzzy logic, notably on extensions of the traditional fuzzy sets, in particular the intuitionistic fuzzy sets, also comprising a considerable part on the generalized nets (GNs) and the intercriteria analysis. The topics of the conference combine systems modeling, systems analysis, broadly perceived operational research, notably optimization, decision-making, and decision support, with various aspects of uncertain and imprecise information and their related tools and techniques.