The advancement of biomedical research, drug development, and understanding of disease susceptibility necessitates the use of models that closely mimic human physiology. Traditional animal models often fail to accurately replicate the complex physiology, immune response, and human pathogenesis, thereby impeding clinical translations. Humanized animal models overcome these limitations by incorporating functional human genes, cells, tissues, or organs. It plays a critical role in the development, evaluation, and optimization of specific therapies such as immunotherapies, including monoclonal antibodies and cell-based treatments like CAR-T cells and checkpoint inhibitors. Advancements in genetic engineering and pluripotent stem cell technologies, such as CRISPR-Cas9 and human-induced pluripotent stem cells (iPSCs), have significantly enhanced the development of these models. Despite the progress, humanized models face challenges like immune rejection, insufficient tolerance, and the risk of tumorigenesis, which must be addressed to optimize their utility in research. This chapter underscores the invaluable role of humanized animal models in bridging the gap between laboratory findings and clinical applications, highlighting their impact on the advancement of personalized medicine and therapeutic innovations, while also considering the ethical and technical challenges inherent in their use.