Time reversal (TR) technique involves recording the signal received from transceivers and playing it backwards for it to refocus back to the source. In this paper, we demonstrate by numerical simulation, the feasibility of tracking a mobile agent indoors using time-reversal of electromagnetic waves. We put a beacon at the center of a 100by100 cell room and was able to locate it with an uncertainty of only 25 cells. The method is robust and can still efficiently localize the source even through room walls. Since the cells can be of arbitrary size, the accuracy can be in the order of centimeters or less, easily breaking the limits of GPS.