Wireless sensor networks (WSNs) continue to be an active research area as the deployment of low cost wireless sensors is a promising technique for various applications such as early warning and alert systems, ecosystem monitoring, warehousing, logistics and surveillance. Sensor data is typically interpreted with reference to a sensor’s location, e.g. reporting the occurrence of an event, tracking of a moving object or monitoring the physical conditions of a region. The process of determining the location of a sensor node in a wireless sensor network, commonly known as localization, is a challenging problem as reliance on infrastructure-based technology like GPS is infeasible due to constraints arising from limited on-board computation power and energy supply, as well as, physical deployment conditions (e.g. indoors or underwater). In this chapter, we focus on range-free distributed localization schemes, in particular, schemes based on hop count that can function under realistic conditions.