GaN High-Electron Mobility Transistors (HEMTs) are well on their way to revolutionizing RF, microwave and millimeter-wave communications and radar systems. GaN FETs are also uniquely poised to have a disruptive impact in electrical power management. In all these applications, device reliability remains a significant concern. As the field has expanded, great progress has recently taken place in understanding GaN transistor degradation, especially under high-voltage stress. Detailed electrical studies coupled with comprehensive failure analysis involving a variety of techniques have revealed a rich picture of degradation. Early studies showed that high voltage degradation of GaN HEMTs was characterized by a critical voltage (Vcrit) at which the device gate current abruptly increases. For stress voltage beyond Vcrit, prominent degradation was observed in the drain current and other electrical parameters of the device. More recently, it has been shown that degradation in the gate current can occur for voltages below the critical voltage suggesting that stress time is a key variable in degradation. Cross-section TEM and planar imaging techniques have shown that high-voltage stress induces prominent structural defects such as grooves, pits and cracks in the GaN cap and AlGaN barrier at the edge of the gate. The evolution of these defects correlates well with that of electrical degradation. Recently, a similar pattern of degradation has been observed under high-power DC and RF stress, although not in a consistent way. A significant recent finding is the role that moisture plays in the formation of these structural defects. This suggests a path for mitigation. Separately from device degradation, a significant anomaly affecting GaN transistors is electron trapping which can severely upset device operation on a wide time domain. This talk will review recent research on the electrical reliability and trapping of GaN HEMTs.