The purpose of this chapter is to critically review theory and research on relevant aspects of the measurement of epistemic cognition, and to highlight the changes that have occurred in this area of investigation in the last decades. To deal with measurement implies addressing theoretical issues, as measurement is strictly related to theory. The chapter first introduces the classic approaches to the study of epistemic cognition. Each approach is characterized in terms of construct definition and related measurement. It will be evident that the construct has been conceived in different ways, and consequently has also been measured through different methods. These are critically reviewed to highlight potentials and limitations. The chapter then introduces more recent approaches that have led to articulating and expanding the construct, as well as to outlining how new approaches to measurement may overcome the limitations of the instruments most commonly used. The issues of the specific level at which to measure epistemic cognition, and the cultural aspects that need to be considered are also presented because of their implications for measurement. Finally, current trends are presented and the features of proposed measures are outlined. The chapter ends with overall implications for theory, research, and practice.