High-efficiency light-emitting diodes (LEDs) have shown to be a promising source for lighting that is energy efficient, cost effective, and environmentally friendly. However, it has been reported that the LEDs suffer from a number of reliability and failure issues when the devices are operating at high temperatures and driven by large currents. This work therefore aims to study the reliability issues and the underlying degradation mechanisms of the LEDs, with the focus on improving the application of LEDs used in consumer lighting. We conducted a detailed study on the temperature-dependency of the light-emitting and current–voltage characteristics of InGaN/GaN blue LEDs which are commonly used as the luminescent source for white lighting. The temperature dependent characteristics of the LEDs reported in this thesis, including significant carrier freezing in temperatures below 250 K, pronounce the carrier trapping in shallow traps when operating high-power LEDs at temperatures below 150 K, and deep trapassisted carrier tunneling enhancement when operating the high-power LEDs at high temperatures. We further conducted current-accelerated experiments to study the lifetime and degradation behaviors of the low-power LEDs. It was found that there is a high correlation between the light emission degradation characteristics of low-power LEDs light emission and the generation-recombination current within the junction of the low-power LEDs, which in turn is governed by the intrinsic defect density in the devices. The stressing results further indicate that the light intensity degraded faster at high temperatures. The behavior of LEDs operating at high temperatures (e.g.>340 K) has shown to have fast light intensity degradation rates, which is found to be very different from when they are operating at room temperature. These results are useful for improving the reliability of LED-based lighting devices. We have also developed a trap-assisted electron tunneling model by considering one dimensional (1-D) multiple square potential wells to explain the abnormal leakage current observed in some LEDs. In this theoretical model, as the energy levels in the trap potential well, the charge therein has a step-like variation as the voltage increases. Therefore, the density of the charges within the trap will also have a step-like variation with the increasing voltage being applied to the LEDs. These effects lead to the observation of plateaus in the measured current-voltage characteristics of the LEDs. The proposed model can be further extended to model other light intensity degradation characteristics of the LEDs, and this will be the objective of future research works.