Rapid growth of a tumor can overwhelm the vasculature that supplies it with nutrients and oxygen. Inside such tumors, cells undergo endoplasmic reticulum stress but can survive such adverse microenvironments by an adaptive mechanism called the unfolded protein response (UPR). X-box binding protein-1 (XBP-1) is a critical transcriptional activator of the UPR and is responsible for regulating the function of genes in cell survival. An unconventional splicing of the XBP-1(U) messenger RNA (mRNA) results in two proteins: XBP-1(S) that is often increased in a variety of human cancers and any translated proteins from the unspliced XBP-1(U) mRNA that acts as a dominant negative of endogenous XBP-1(S) action. In cancer cells, overexpression of XBP-1 can confer drug resistance by preventing drug-induced cell-cycle arrest and mitochondrial permeability and apoptosis, while downregulation of XBP-1 increases the sensitivity to killing by hypoxia. XBP-1 is also implicated in cellular de-differentiation, oncovirus infection and the epitheliat-to-mesenchymal transition. Given that XBP-1 mediates a wide range of responses in tumorigenesis, it is logical to focus on XBP-1 as an anticancer therapeutic target. Furthermore, combining inhibitors of XBP-1 with other anti-UPR drugs may enhance the activity of some antineoplastic therapies.