Brain tumors are complex cellular ecosystems, composed of populations of both neoplastic and non-neoplastic cell types. While the contributions of the cancer cells in low-grade and high-grade gliomas have been extensively studied, there is comparatively less known about the contributions of the non-neoplastic cells in these tumors. As such, a large proportion of the non-neoplastic cells in gliomas are resident brain microglia, infiltrating circulating macrophages, and T lymphocytes. These immune system-like stromal cells are recruited into the evolving tumor through the elaboration of chemokines, and are reprogrammed to adopt new cellular identities critical for glioma formation, maintenance, and progression. In this manner, these populations of tumor-associated microglia and macrophages produce growth factors that support gliomagenesis and continued tumor growth. As we begin to characterize these immune cell contributions, future therapies might emerge as adjuvant approaches to glioma treatment.