Cancer immunology is among the most important disciplines of cellular and molecular immunology, and it has progressed significantly in the last decade. A tumor is a mass of tissue that develops as a result of uncontrolled cell division. The ability of tumor cells to proliferate and invade distant areas is the key feature of tumor cells. In this regard, the immune system plays an important role in the suppression or progression of tumor growth. The immune microenvironment around the tumor consists of a heterogeneous population of immune cells with specific roles. Some of them, such as myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), and M2 macrophages, through the expression of immunosuppressive molecules, could exert a protumor role, leading to cancer progression. In contrast, several types of immune cells, such as cytotoxic T lymphocytes (CTL) and natural killer cells (NK), fight against tumor development by exerting antitumor mechanisms. Therefore, understanding the function of immune system arms might help in determining antitumor responses or even therapeutic options. This chapter provides an overview of the immune system’s role, as well as key immune system players in the tumor microenvironment. It also provides some of the most common immune-based therapies.