Brain metastasis (BrM) diagnosis represents the most devastating complication of advanced cancer, with very few effective treatments. As the BrM tumor microenvironment (TME) has gained attention in the emergence of immunotherapeutic drugs—currently, most BrM patients have not experienced clinical benefits—underscoring the knowledge gap regarding immune escape mechanisms at the metastatic site. Therefore, a deeper understanding of the immune cell composition, tumor-immune cellular networks, and immune escape mechanisms within BrM is essential to advancing more precise immunotherapeutic approaches.