A system is configured to delivering radiofrequency power to the endometrial lining tissue of a uterine cavity, including modulating the delivered power so that a measured impedance of the endometrial lining tissue tracks a target impedance as a function of time, wherein the target tissue impedance is derived from a function that approximates a preferred endometrial lining tissue ablation impedance curve that is determined based upon a measured impedance of the endometrial lining tissue after RF power has been delivered for a predetermined initial time period.
