Tumor immune escape is now a hallmark of cancer development, and therapies targeting these pathways have emerged as standard of care. Specifically, immune checkpoint signal blockade offers durable responses and increased overall survival. However, the majority of cancer patients still do not respond to checkpoint blockade immune therapy leading to an unmet need in tumor immunology research. Sex-based differences have been noted in the use of cancer immunotherapy suggesting that sex hormones such as estrogen may play an important role in tumor immune regulation. Estrogen signaling already has a known role in autoimmunity, and the estrogen receptor can be expressed across multiple immune cell populations and effect their regulation. While it has been well established that tumor cells such as ovarian carcinoma, breast carcinoma, and even lung carcinoma can be regulated by estrogen, research into the role of estrogen in the regulation of tumor-associated immune cells is still emerging. In this chapter, we discuss the role of estrogen in the tumor immune microenvironment and the possible immunotherapeutic implications of targeting estrogen in cancer patients.