In this chapter, I provide argument and evidence that the scope of people's ignorance is often invisible to them. This meta-ignorance (or ignorance of ignorance) arises because lack of expertise and knowledge often hides in the realm of the “unknown unknowns” or is disguised by erroneous beliefs and background knowledge that only appear to be sufficient to conclude a right answer. As empirical evidence of meta-ignorance, I describe the Dunning–Kruger effect, in which poor performers in many social and intellectual domains seem largely unaware of just how deficient their expertise is. Their deficits leave them with a double burden—not only does their incomplete and misguided knowledge lead them to make mistakes but those exact same deficits also prevent them from recognizing when they are making mistakes and other people choosing more wisely. I discuss theoretical controversies over the interpretation of this effect and describe how the self-evaluation errors of poor and top performers differ. I also address a vexing question: If self-perceptions of competence so often vary from the truth, what cues are people using to determine whether their conclusions are sound or faulty?