A detailed model for the reset process kinetics in HfO 2 -based RRAM is presented describing the transition between low and high resistance states at the atomic level. Based on the filament characteristics as observed by TEM, the kinetics of the reset operation is simulated using the Time Dependent Monte Carlo (TDMC) method incorporating ab-initio calculated microscopic characteristics of the oxygen ions in hafnia. Temperature and field driven oxygen diffusion in the oxide surrounding the filament is shown to provide the needed supply of oxygen to re-oxidize the tip of the filament and switch the device to the High Resistance State (HRS).