The unfolded protein response (UPR) is a cellular stress response pathway that is activated when cells are under conditions of endoplasmic reticulum (ER) stress, such as the accumulation of misfolded or unfolded proteins. The UPR is a highly conserved pathway that aims to restore protein folding homeostasis in the ER by increasing the capacity of the cell to fold and degrade proteins. The UPR involves three main signaling pathways that are initiated by three ER transmembrane sensors: inositol-requiring enzyme 1 (IRE1), protein kinase RNA-like ER kinase (PERK), and activating transcription factor 6 (ATF6). These sensors detect the accumulation of misfolded proteins in the ER and activate downstream signaling cascades to alleviate ER stress. The UPR regulates a variety of cellular processes, including protein synthesis, transcription, and apoptosis, to restore ER homeostasis. Dysregulation of the UPR has been implicated in a variety of human diseases, including neurodegenerative diseases, metabolic disorders, and cancer. Understanding the mechanisms by which the UPR is activated and regulated may provide insights into potential therapeutic strategies for these diseases. Overall, the UPR is a critical cellular pathway that plays a key role in maintaining protein folding homeostasis and cellular function under conditions of stress.