We propose a new localization method in order to estimate the position of wireless devices in real time, using already existent infrastructure (such as IEEE 802.11 access points). We present a filtering algorithm that computes the most likely path taken by a mobile node, based on signal strength measure- ments and a known signal strength map. In our approach we do not assume that the node mobility model is known. The algorithm calculates analytically in real time the optimal correction in the mobile node's estimated position, using current signal measurements. We discuss how this algorithm can be used in a practical implementation and we evaluate the performance of our method using numerical simulations.