We have systematically studied both the spontaneous and stimulated emission properties in blue-light-emitting In(x)Ga(1-x)N/GaN multiple quantum well structures using various linear and nonlinear optical techniques. Our experimental observations are consistently understandable in the context of localization of carriers associated with, large potential fluctuations in the In(x)Ga(1-x)N active regions and at heterointerfaces. The studies have been done as a function of excitation power density, excitation photon energy, excitation length, and temperature. The results show carrier localization features for spontaneous emission and demonstrate the presence of potential fluctuations in the In(x)Ga(1-x)N active region of the In(x)Ga(1-x)N/GaN structures and its predominant role in spontaneous emission. In addition, the experimental observations strongly indicate that the stimulated emission has the same microscopic origin as spontaneous emission, i.e., radiative recombination of localized states. Therefore, we conclude that carriers localized at potential fluctuations in In(x)Ga(1-x)N active layers and interfaces can play a key role in not only spontaneous but also stimulated emission of state-of-the-art blue-light-emitting In(x)Ga(1-x)N/GaN quantum structures.