This chapter presents the RILEM (International Union of Laboratories and Experts in Construction Materials) recommendations for using recycled aggregates, particularly Recycled Concrete Aggregates (RCA), in new concrete. It outlines different classifications of recycled aggregates (Type I, Type II, and Type III), based on their material composition (masonry rubble vs. concrete rubble) and provides specifications for their usage in concrete mixtures. The chapter also discusses the mechanical properties of concrete made with recycled aggregates and their limitations, such as the impact on strength and durability. This chapter emphasizes the importance of proper classification and standards to ensure the quality and safety of recycled concrete in structural applications.