Immune cells of different lineages in the tumor microenvironment (TME) play essential roles in tumorigenesis. Some cells act against the tumor, while others show tumor-supporting functions. During early tumorigenesis, cells with anti-tumor activity attempt to target and kill the cancer cells.. However, through evasion of immune surveillance and/or inhibition of immune cell cytotoxicity, cancer cells are able to escape cell death. Paradoxically, this immune evasion capacity provides an opportunity for research and development of new cancer therapy strategies, thus taking advantage of the characteristics and capabilities of immune cells to attack cancer cells. The principal intention of this chapter is to give the reader a synopsis of the immune cell biological functions within the TME and discuss their influence in cancer immunotherapy and outline prospects of future research that will contribute to more insight into the functions of immune cells that would open new opportunities for future therapies. An example of this is the use of therapies that block immune checkpoints (anti-CTLA4, anti-LAG3, and anti-PD antibodies) or direct immune cells (CAR-T). These anti-cancer therapies have shown significant anti-tumor effects in multiple cancers, thus ushering in a new era in cancer therapy.