This chapter investigates the microstructure and nanomechanical properties of interfacial transition zones (ITZs) in recycled aggregate concrete (RAC) using nanoindentation and atomic force microscopy (AFM). The study focuses on the effects of mixing approaches, hydration age, and aggregate type on the properties of old and new ITZs. Results show that the thickness of old ITZs is around 40–50 µm, while new ITZs are thicker at 55–65 µm. The average modulus of old ITZs is 70–80% of the old paste matrix, and new ITZs exhibit 80–90% of the modulus of the new paste matrix. The Two-Stage Mixing Approach (TSMA) improves the nanomechanical properties of new ITZs by reducing porosity compared to the Normal Mixing Approach (NMA). The study also finds that hydration age significantly affects the properties of new ITZs, with modulus increasing and thickness decreasing over time. The properties of old ITZs remain relatively stable with age. These findings provide insights into improving the mechanical behavior of RAC for broader applications.