This chapter provides a comprehensive overview of the properties, production technologies, and regional variability of recycled aggregates (RA), particularly recycled coarse aggregates (RCA). It explores the influence of adhered mortar (AM), impurities, particle shape, and production methods on physical and mechanical characteristics such as absorption, density, strength, and abrasion resistance. The chapter also highlights regional practices, repeated recycling effects, and quality control standards across different countries. Through extensive literature and experimental data, it establishes a detailed understanding of the material behavior of RA, forming the basis for evaluating its use in concrete.