<p>Drug resistance, which arises from a variety of biological mechanisms such as drug efflux by ATP-binding cassette transporters, epithelial-to-mesenchymal transition (EMT), the presence of cancer stem cells (CSCs), immune system evasion, and changes in the tumor microenvironment (TME), poses serious treatment challenges for breast cancer. This paper offers a thorough analysis of various resistance mechanisms, going into great detail into the molecular, cellular, and genetic contributors. We next look at how nanotechnology-based approaches, such multifunctional nanocarriers, smart nanoparticle systems, and targeted drug delivery, can be developed to get around these resistance mechanisms, building on this biological basis. The use of particular nanotechnology platforms (such as liposomes, micelles, and dendrimers) to target CSCs, circumvent efflux pumps, alter immunological responses, and improve treatment precision is highlighted. Important issues in clinical translation and regulatory considerations are also covered in the study. This two-part investigation seeks to improve therapeutic outcomes for drug-resistant breast cancer by bridging the gap between molecular insight and nanotechnological innovation.</p> Graphical Abstract <p></p>