Mononuclear phagocytes are versatile cells that can express different functional programs in response to microenvironmental signals. Fully polarized M1 and M2 (or alternatively activated) macrophages are the extremes of a continuum of functional states. Macrophages that infiltrate tumor tissues are driven by tumor-derived and T cell-derived cytokines to acquire a polarized M2 phenotype. These functionally polarized cells, and similarly oriented or immature dendritic cells present in tumors, have a key role in subversion of adaptive immunity and in inflammatory circuits that promote tumor growth and progression.