Hormone-receptor-positive breast cancer accounts for roughly 70% of diagnoses, yet long-term control is undermined by endocrine resistance driven by ESR1 mutations, adaptive signaling crosstalk, and sanctuary-site metastasis. Nanotheranostics platforms, which are nanoscale constructs that incorporate both imaging agents and endocrine payloads, provide an innovative approach to enhancing hormone therapy. They enable real-time visualization coupled with precise drug delivery. Polymeric micelles, lipid nanoparticles, and superparamagnetic iron-oxide cores have been engineered to ferry selective oestrogen-receptor degraders (SERDs), such as elacestrant, approved in 2023 for ESR1-mutated metastatic disease, deep into tumor and bone-marrow niches while simultaneously enhancing MRI, PET, or photoacoustic contrast. Acid-labile or enzyme-cleavable linkers trigger drug release only within the acidic or protease-rich tumor microenvironment, minimizing off-target exposure that underlies osteoporosis, thromboembolic events, and cardiometabolic toxicity seen with free endocrine drugs. Beyond single-agent therapy, multifunctional surfaces accommodate aromatase-inhibitor prodrugs, siRNA that silences mutant ESR1, and immunomodulatory ligands, enabling concerted attacks on both tumor cells and stromal resistance loops. Magnetic or acoustic guidance further concentrates these hybrids in brain and bone lesions, historically impervious to conventional endocrine agents. In pre-clinical models, such “all-in-one” nanocarriers deliver ≥10-fold higher intratumoral SERD levels, restore endocrine sensitivity, and allow longitudinal imaging to verify target engagement. At the same time, early human pilot studies indicate improved lesion conspicuity and acceptable safety. This chapter traces the evolution of nanotheranostics for hormone therapy from materials engineering and resistance-targeted payload design to imaging physics, translational milestones, safety considerations, and future clinical integration providing a concise roadmap for researchers and clinicians seeking to harmonize diagnosis and endocrine intervention in breast cancer.