This paper reports an extensive analysis of the electrostatic discharge (ESD) robustness and of the corresponding failure mechanisms of AlGaN/gallium nitride high-electron mobility transistors. Based on transmission line pulse testing, we demonstrate the following results: 1) when ESD pulses are applied to the drain under pinchoff conditions, failure occurs through a field-dependent mechanism; the failure voltage is independent on the gate bias, and approximately equal to 300 V; 2) when ESD pulses are applied to the drain under ON-state conditions, a power-dependent mechanism prevails, lowering the ESD robustness; 3) vertical (drain substrate) breakdown does not significantly affect the ESD stability of the devices; the failure threshold for vertical failure is higher than 450 V; and 4) the length of the field plate can significantly impact the ESD behavior of the devices, by influencing the failure threshold and by favoring soft-degradation processes. Scanning electron microscopy was extensively used to achieve a complete description of the failure processes. © 2015 IEEE.