Cancer cells are exposed to various intrinsic and extrinsic factors that disrupt protein homeostasis, producing endoplasmic reticulum (ER) stress. To cope with these situations, cancer cells evoke a highly conserved adaptive mechanism called the unfolded protein response (UPR) to restore the ER homeostasis. Recently, several pharmacological agents have been found to exhibit anti-tumor activity by targeting the UPR components. The development of potent and specific compounds that target the UPR components has not only shed light on the regulation of the UPR in cancer cells, but also brought the field closer to clinical drug candidates. Here we present an overview of the milestones in the field of UPR biology in cancer with a focus on new strategies for pharmacological inhibition.