This textbook is for use in tutorials and seminars by psychology, neuroscience and cognitive science undergraduates studying cognition. The book complements standard course texts in cognition by providing a series of articles which emphasize particularly what we do not understand, rather than what we think we do. It considers a selection of problems and phenomena that remain mysterious despite years, decades or centuries of enquiry, and evaluates different approaches to these problems.; The topics discussed range from specific optical illusions to the nature of consciousness. Some of these unsolved problems provide a vehicle for reviewing different paradigms and shifts in the field over the 20th century. Each chapter also poses some of the remaining unanswered questions, suggesting directions for future enquiry.