Curiosity is a well-known but under-researched affective construct in psychology and mathematics education. In this chapter, we reflect on the definition of curiosity in the context of the key characteristics of affect and discuss how curiosity is anchored in theories of motivation and emotions. Furthermore, we analysed the differences and similarities between curiosity and interest in mathematics to contribute to the conceptual clarity of curiosity. After these considerations, we present previous findings on the effects of curiosity on learning outcomes as well as on the effects of interventions that were aimed at increasing curiosity in mathematics and other domains. In addition, we summarise some promising interventions that were found to increase students’ interest and are worthy of investigation for their potential positive effects on curiosity. In the final section, we outline directions for future research on curiosity, such as the need to investigate differences in curiosity with regard to various mathematical topics, real-world tasks, and specific contexts.