Recent observations have revealed the existence of infrared brightening in some nova explosions, and its absence in others. These infrared excesses are ascribed to thermal emission from grains which are considered to consist of graphite. Such nova explosions are widely accepted to be triggered by hydrogen shell-flashes on the surface of white dwarfs which accrete matter in close binary systems. As for the hydrogen shell-flash, recently, a general theory applicable even to the case of finite amplitude has been developed. According to this theory, the progress of a shell-flash is determined only by the mass of the white dwarf MWD and the mass of the accreted hydrogen-rich envelope ΔMH.