The sensor localization problem is the problem of finding the best location estimation of radiating sensors, from several noisy range measurements of distances collected using a network. Each sensor has a power supply and a radio transceiver that enable it to monitor its immediate vicinity. In this project, we consider two types of problems: the first one refers to the problem of finding the best estimate for the location of only one radiating source this type of problem is known in the literature as Single Source Localization (SSL) problem. For such problems, we assume that the location of the anchors are known exactly. In the second type, we deal with networks with a large number of non-anchor sensors, this type of problem is referred to as Wireless Sensor Network Localization (WSNL) problem. In this work we tackled both cases by designing novel range-based algorithms that utilize Time-of-Arrival information, in order to determine the position of each of the sensors.