Many innate and adaptive immune cells are present in endometrial cancer (EC) and play key roles in the tumour microenvironment (TME); through initiation, stimulation, and modulation of the surrounding environment, these immune cells can promote or suppress the development of EC. Their impact on the TME is closely linked to their prognostic significance, and several immune cells can be associated with a positive or negative prognosis dependent on their affect, tumour type, and tumour grade. Regarding the innate immune response, an overall positive prognosis can be associated with decreased numbers of dendritic cells (DCs) and an increased neutrophil and NK cell number. In contrast, a negative prognosis can be linked to increased macrophage and mast cell density within the tumour. Eosinophils in the TME can be correlated with either a good or poor prognosis because they are heavily cancer type and grade specific. Other important factors also have significance and are considered, including spatial distribution, influential molecules, and phenotypic variation. Whereas in adaptive immunity, T cells are phenotype dependent regarding prognostic significance; they display both pro- and antitumour characteristics. However, B cells are linked to a poor prognosis, but limited research has been done to support this. This chapter explores roles of immune cells and their components in prognostic significance of EC.