GRP78/BiP is a major endoplasmic reticulum (ER) chaperone protein critical for protein quality control of the ER, as well as controlling the activation of the ER-transmembrane signaling molecules. Through creation of mouse models targeting the Grp78 allele, the function of GRP78 in development and disease has been investigated. These led to the discovery that GRP78 function is obligatory for early embryonic development. However, in adult animals, GRP78 is preferably required for cancer cell survival under pathologic conditions such as tumor progression and drug resistance. The discovery of surface localization of GRP78 in cancer cells reveals potential novel function, interaction with cell-surface receptors, and possible therapeutic implications. Mouse models also reveal that GRP78 controls maturation and secretion of neuronal factors for proper neural migration and offers neuroprotection. Antioxid. Redox Signal. 11, 2307–2316.