Cancer is a major worldwide health issue that spans ethnic backgrounds, age groups, and socioeconomic status. Although therapeutic advances have revolutionized cancer treatment and significantly enhanced patient survival, cancer remains a leading cause of mortality across the globe. Despite the implementation of targeted therapies, patients often relapse and develop resistance, thus prompting the need to investigate alternate avenues for cancer treatment. The clinical success of immune checkpoint inhibitors in melanoma and lung carcinoma has encouraged efforts to evaluate immunotherapy across various tumor types, such as breast cancer. Unfortunately, these trials have demonstrated only a modest clinical benefit in many scenarios. Therefore, it is essential to understand normal immunity and elucidate resistance mechanisms that lead to cancer progression to develop novel therapeutic strategies that augment antitumor immunity in more refractory tumors. In this chapter, we summarize normal immune system function, interactions between tumors and the host immune system, and methods tumors use to evade host immunity resulting in progression.