The discovery of an agent that selectively kills tumor cells and not normal cells is the dream of every cancer researcher. Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), first discovered in 1995, was heralded as a selective killer of tumor cells, and its potential is still thought to be high. Almost immediately, broad efforts were made to understand its activity at the molecular level. TRAIL has been shown to interact with the cell surface through five distinct receptors, named death receptor (DR) 4, DR5, decoy receptor (Dc)R1, DcR2, and osteoprotegrin. It activates nuclear factor (NF)-kappaB, c-Jun N-terminal kinases, and apoptosis. The apoptotic signals are mediated through Fas-associated death domain protein (FADD)-mediated recruitment of caspase-8 and caspase-3. Additionally, caspase-8 can cleave Bcl-2 homology domain 3 (BH3)-interfering domain death agonist (Bid), and the cleaved Bid then causes the release of mitochondrial cytochrome c, leading to the activation of pro-caspase-9, which can then activate pro-caspase-3. TRAIL-induced apoptosis is negatively regulated by numerous cellular factors including decoy receptors, cellular FADD-like interleukin 1 beta-converting enzyme (FLICE) interacting protein (cFLIP), cellular inhibitor of apoptosis protein (cIAP), X-linked IAP (XIAP), survivin, and NF-kappaB. Second mitochondria-derived activator of caspases (Smac)?direct IAP binding protein with low pI (DIABLO) mediates proapoptotic signals through inaction of IAP. How the TRAIL-induced apoptosis is downregulated by these factors is discussed in detail in this review. Whether TRAIL selectively kills tumor cells without harming normal cells is also discussed.