I present the origin of operational instability in the quantum dot (QD) based light-emitting diodes (QLEDs). From the comparative analysis of electrical and optoelectronic characteristics of QLEDs in relationship with photophysical properties of the QD emissive layers, I uncover that the device degradation occurs via two distinct processes. The first is the luminescence efficiency drop of the QD emissive layer as a result of accumulation of excess charge carriers in QDs. The other is the electron leakage toward the hole transport layer (HTL) that accompanies irreversible HTL’s degradation. These two main mechanisms appear to contrast in respects to the time scale and the reversibility, but have a single origin, the imbalance between electron and hole injection rates into QDs. The understanding gained from the present study offers practical guidelines that promise the realization of high performance QLEDs with proven operational stability.