Research about learning reveals that learning is not merely a result of planned activities carried out within the boundaries of the classroom but rather it is a comprehensive concept that incorporates a wide range of experiences that take place both inside and outside the classroom (Dewey, 1938; Falk & Dierking, 2000). This chapter focuses on out-of-school learning and serves as a topical introduction to this book. Out-of-school learning activities, which include experiences organized with community partners and reflect everyday mathematics used across life, may be carried out inside the school but outside the classroom or outside the school (Barron, 2006; NRC, 2009). In other words, the construct of out-of-school learning captures the use of mathematical knowledge or concepts in education that are found and used in out-of-school environments (zoos, botanical gardens, museums, historical sites, culturally significant settings, etc.) or in school spaces which might nonetheless be labeled as out-of-classroom environments (school garden, corridors, laboratories, sports hall, etc.) (Beames et al., 2012; Schneiderhan-Opel & Bogner, 2021). Consequently, out-of-school mathematics activities have an interdisciplinary flavor, similar to STEM activities that utilize out-of-school environments bringing together various disciplines, such as science centers, planetariums, and botanical gardens (Falk & Needham, 2011; Ito et al., 2013). In out-of-school learning contexts, students can actively use the theoretical mathematical knowledge they have learned at school to solve authentic problems within a real-world context (Fredricks & Eccles, 2006). We begin this chapter with an exploration of the relationship between out-of-school learning and formal, informal, and non-formal learning. We review the basic characteristics of out-of-school learning activities, emphasizing the importance of out-of-school learning environments in terms of mathematics teaching. We also discuss which places can be used as out-of-school learning environments (Gutiérrez & Rogoff, 2003; Aslan & Arabacı, 2023). We then go into some detail about the most significant advantages and some of the challenges of facilitating out-of-school learning of mathematics. We end the chapter with an overview of measurement and evaluation techniques specific to out-of-school learning environments (Bouffard & Little, 2004; Krajcik & Blumenfeld, 2006).