In recent years, ignorance has been reevaluated as a concept of significant importance in many areas of research. Emerging interdisciplinary research, most notably within the growing field of Ignorance Studies, has recently proposed that ignorance is a cognitive phenomenon worthy of attention and that can be structured, motivated, productive, and even adaptive. Philosophical and psychological research are uniquely positioned to explore the complexities of ignorance’s impact on agents’ cognition. This entry aims to explore the psychological dimensions of ignorance by examining its various forms, functions, and consequences from a theoretical perspective. Drawing on recent literature and using the well-known Rumsfeld Matrix as a compass—with which it is possible to distinguish between Known Knowns, Known Unknowns, Unknown Knowns, and Unknown Unknowns—this entry posits that ignorance is not merely a gap in what is known but a dynamic and meaningful condition that shapes how individuals think, feel, and act.