That we as humans do make errors in thinking, judgment, and memory is undisputed. In fact, there is a plethora of phenomena showing that we deviate in our thinking, judgment, and memory from some objective and arguably “correct” standard. However, trying to provide a complete list of these biases and fallacies would be a tedious task. I therefore restricted the selection of cognitive illusions in this book to those that are more well known and that have inspired a wealth of empirical studies and theoretical approaches in order to better understand the underlying mechanisms of the human information-processing system or to dispute their status as a “bias” by questioning the normative standards of comparison and the applied experimental methods. This introductory chapter gives an overview about cognitive illusions in general, discusses their theoretical status, and describes the intentions of this book.