Stellar explosions mark deaths of stars. Both theory and observations indicate that at least two different types of stellar explosions exist in nature: Type Ia supernovae produced by thermonuclear explosions of white dwarfs, and core-collapse supernovae that are believed to be produced by neutrino winds from the collapsing core in massive stars (M > 8 Msun). Recent observational surveys have found an enormous diversity of stellar transients, which cannot be easily explained by the standard theory of stellar evolution. They include super-luminous supernovae and supernova imposters. In this talk, I will briefly review the supernova theory and discuss recent efforts for a better understanding on supernova diversity. In the last part, I will present my own recent project on the search for signatures of pair-instability supernovae, which have been predicted by theory but never discovered by observations.