In this introductory chapter, I describe what constitutes the domain of cognitive illusions and how cognitive illusions can be characterized. Much debate has evolved around the classical “heuristic and biases” approach, also involving the accusation of producing more or less artificial biases. I discuss these arguments in detail including the question of which normative standard to apply and conclude that cognitive illusions span a wide range of phenomena from simple “tricks” up to hard-wired and unavoidable systematic errors. Next I discuss potential explanations, especially dual-process models and simple information-processing strategies. Finally, I take a look at the impact of cognitive illusions in applied contexts outside the lab and how the adaptive value of cognitive illusions can be judged. Most researchers probably agree that cognitive illusions can be seen as the (mostly bearable) costs of an otherwise highly functioning and efficient cognitive system.