The understanding that immune cells in, or missing from, the tumor microenvironment (TME) are a central part of cancer progression is now well established. However, in part because these immune cells vary so much in types, numbers and function, there are certainly large gaps in both general and detailed understanding of their roles. Having both broader and deeper understanding of the role of immune cells in the TME will generate better understanding of cancer biology and importantly, will lead to new options for treating tumors with various therapy strategies, including immunotherapy. Additionally, many studies have linked immune TME infiltrate data in specific cancers to patient outcome and therapy responses, which can guide treatment application. Overall, the current phase of cancer research is heavily focused on the TME and we are confident this Special Issue will help expand the understanding of immune cells in the TME. The topic is broad, and we anticipate publishing papers on a wide range of subjects grouped within the "Role of immune cells in the tumor microenvironment".