The immune response in the tumor microenvironment is complex, consisting of cells from both the adaptive and innate immune systems. The phenotype and function of these cells are dictated by cytokines present in the microenvironment, as well as by the interactions of these cells with the tumor cells and each other. Technological advances have allowed investigators to better identify the specific immune cells present and immune-related gene signatures overexpressed in the tumor microenvironment. Increased knowledge of tumor immunology has allowed us to better understand how these cells and the developing tumor interact. Together, these advances have prompted the conduct of numerous studies investigating the prognostic and predictive significance of immune infiltrates.