Breast cancer (BC) remains one of the most frequently diagnosed and life-threatening malignancies among women worldwide. However, current therapeutic strategies are often hindered by systemic toxicity, multidrug resistance, and insufficient targeting efficiency. The advent of nanotechnology has introduced metallic nanoparticles (MNPs) as promising tools in both the diagnosis and treatment of BC. Because of their unique physicochemical properties, such as high surface area, tunable size, surface modifiability, and distinctive optical, magnetic, or catalytic features, MNPs have shown significant potential to overcome the limitations of conventional therapeutic and diagnostic approaches. This chapter provides a comprehensive overview of recent advancements in the application of various MNPs—including gold (Au), silver (Ag), iron oxide (Fe₃O₄), zinc oxide (ZnO), titanium dioxide (TiO₂), and copper oxide (CuO)—for the diagnosis and treatment of BC. Their roles in enhanced imaging techniques (e.g., MRI, CT, photoacoustic imaging), targeted drug delivery, gene therapy, photothermal, and photodynamic therapies are critically examined. Emphasis is placed on their ability to facilitate tumor-specific targeting, improve therapeutic efficacy, and minimize systemic side effects. Furthermore, the chapter discusses diagnostic and therapeutic strategies with MNPs and the synergistic integration of diagnostic and therapeutic functionalities. Despite their promise, challenges related to toxicity, biodegradation, pharmacokinetics, and regulatory considerations remain significant barriers to clinical translation. Therefore, ongoing research is required to optimize nanoparticle design for safe, effective, and personalized BC care. This chapter highlights recent advances in the utilization of MNPs and their surface-functionalized variants for the diagnosis, targeted therapy, and theranostic approaches in BC management.