While the failure to answer political knowledge questions in opinion surveys has traditionally been interpreted as a consequence of a lack of information and knowledge, the idea that people might also fail to answer knowledge questions because they firmly believe in the wrong answer and hold misperceptions has gained ground. There is now a broad consensus about the importance of conceptually differentiating between those who lack knowledge – the uninformed – and those who hold misperceptions – the misinformed. Differentiating between these two groups empirically in survey-based research has however proven difficult. There are also several examples of how the same measures sometimes are used to identify the uninformed, sometimes to identify the misinformed. Against this background, this chapter first reviews and provides an overview of how misperceptions and lack of knowledge have been conceptually and empirically differentiated in extant research. Based on this, the chapter offers several suggestions for how to differentiate the uninformed from the misinformed in future survey-based research.