Leukemias comprise a family of hematologic malignancies characterized by the over-proliferation of myeloid precursor cells. Leukemia development depends on multiple factors, including supportive growth signals from non-leukemia cells in the surrounding tumor microenvironment. Identifying these critical supportive cells, and eliminating their support, remains challenging. This dissertation contains novel research that identifies supportive monocyte/macrophage cells expressing colony stimulating factor 1 receptor (CSF1R) in human chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML).