Emergency services in indoor environments can take advantage of local positioning systems. A simple but reliable localization system based on passive RFID technology is here proposed. It resorts to ad-hoc designed antennas for tags, low radiated power and simple localization algorithms. A theoretical model of the system has been developed and tested by means of numerical and experimental analysis. The system allows a moving user handling an interrogating device to localize himself with respect to a grid of fixed tags used as reference anchors. Experimental results show both the feasibility and the reliability of the proposed system.