Primary brain tumors, such as gliomas, and secondary brain metastases remain generally lethal. Seminal contributions during the past decade unveiled the intricacies that govern the immune microenvironment of these tumors in the brain. These discoveries highlighted how cancer cells hijack the immune regulatory mechanisms in the brain to promote immunosuppression and tumor progression. This chapter explores the cellular and molecular networks of the immune system in the setting of brain tumors. Interactions between brain tumors and tumor-associated macrophages and microglia, neutrophils, lymphocytes, and astrocytes are described, shedding light on potential areas for immunotherapeutic intervention in the future.