Hypoxia is a dynamic feature of the tumor microenvironment that contributes to cancer progression. In order to adapt and overcome hypoxic stress, tumor cells activate survival pathways that attempt to couple metabolic processes to reduced energy availability due to oxygen deprivation. While the hypoxia-inducible factors HIF-1 and HIF-2 are critical to the cellular response to hypoxia, HIF-independent processes are known to contribute to this adaptation. Recent evidence demonstrates that hypoxia activates components of the Unfolded Protein Response (UPR), a coordinated program that regulates cellular adaptation to increased levels of unfolded proteins in the endoplasmic reticulum (ER). Here we review the evidence implicating the ER kinase PERK, its downstream target translation initiation factor eIF2alpha, and the subsequent translational upregulation of the transcription factor ATF4 in this response. Not only are cells with compromised PERK-eIF2alpha-ATF4 signaling more sensitive to hypoxic stress in vitro but they also form tumors that grow slower in vivo with smaller hypoxic areas, indicating that the PERK-eIF2alpha-ATF4 pathway confers a survival advantage for tumor cells under hypoxia. These results, together with evidence for an involvement of other UPR pathways and ER stress proteins in hypoxia tolerance and tumor maintenance, point to a central role for UPR activation in tumor progression and suggest that this response may offer an attractive target for new anti-tumor modalities.