Antibody-based therapies entered the clinic over 30 years ago and have become a mainstream therapeutic option for patients with malignancies, infectious diseases, and transplant rejection. Compared with traditional chemotherapy, these biotherapeutics preferentially target cells presenting tumor-associated antigens, resulting in improved treatment outcomes and reduced side effects. Despite their excellent selectivity and a broad collection of targets, the therapeutic effects of monoclonal antibodies can decrease over time due to the development of immune resistance and suppression by the immune system. To circumvent the internal resistance and boost the therapeutic effects, the modification of antibodies by various chemical molecules (e.g., drugs, nanoparticles) or biological reagents (e.g., enzymes, cytokines, other antibodies) is required. To covalently label antibodies, various methods have been developed, most commonly involving nonspecific reactions on lysine and cysteine residues. The resulting products are heterogeneous antibody conjugates which may suffer from diminished binding affinity and therapeutic index due to a lack of control over the modification ratio and sites. With the development in the fields of biorthogonal chemistry and protein engineering, site-specific antibody conjugation strategies have gone mainstream and predominantly been used for clinical treatment. The leading site-specific antibody conjugation techniques on the market, including THIOMABTM, SMARTagTM, SiteClickTM, have been proved successful. However, all these site-specific antibody conjugation methods require a certain amount of antibody engineering, which is time-consuming, expensive, and may result in low yield. Therefore, antibody scientists have a craving for the next generation of site-specific antibody conjugation methods without antibody engineering and/or UV/chemical treatment. In this dissertation, I describe how we develop a proximity-induced antibody conjugation method and how we apply the method for cancer and bone metastasis treatment. First, to suppress all the limitations mentioned above, we developed a new platform for efficient and site-specific labeling of native antibodies based on proximity-induced reactivity between a non-canonical amino acid (ncAA) and a nearby antibody lysine residue. The resulting proximity-induced conjugation technology, named pClick, does not require any antibody engineering or UV/chemical treatment, thus enabling attachment of various functional molecules to most antibodies used for research and therapy. By using this method, we collaborate with the Wistar Institute to conjugate sialidase to HIV broadly neutralizing antibodies. These conjugates selectively desialylated HIV-infected cells and enhanced natural killer cells (NK cell) capacity to kill infected cells. Second, despite the great conjugation efficiency of pClick to intact antibodies, the relatively large size and low production yield of FPheK-containing FB protein prepared by Genetic Code Expansion greatly limits the further application of this method. To expand the application of pClick, we developed a proximity-induced site-specific antibody conjugation method using solid-phase synthesized antibody affinity peptide (pClick2.0). To illustrate the utility of this concept, we have prepared well-defined antibody-drug conjugates (ADCs) and bispecific antibody (bsAb) conjugates. The resulting conjugates exhibit excellent cytotoxic anctivity against cancer cells in vitro and superb anti-tumor activity in mouse xenograft models. Third, we demonstrate the potential of combining antibody engineering and antibody conjugation method (pClick2.0) by constructing a bone-homing antibody-drug conjugate with a moderate bond-targeting capability which exhibited great efficacy to inhibit breast cancer metastases as well as multiorgan secondary metastases in xenograft models. This methodology establishes a new strategy for transitioning antibody-based therapies from antigen-sepcific to both antigen- and tissue-specific, thus providing a promising new avenue for advancing antibody therapy toward clinical translation. In summary, the work in this dissertation has shown advances in different aspects of current obstacles in site-specific antibody conjugation technology.