Security of water distribution networks (WDNs) through early warning systems is one of the topics of research over the last two decades to safeguard human health and the environment against accidental and intentional contamination. Several methodologies have been suggested by different researchers for placement of sensors in WDNs which can provide early warning. Owing to the cost of both placing and maintaining the sensors, the number of these must be limited. This constraint makes the sensor deployment locations crucial in early warning systems. In this paper various methodologies suggested for sensor placements have been broadly classified based on the number of performance objectives as single and multi-objective sensor location problems. Various objectives and their significance in the context of sensor placement strategy have been discussed. A complete review of literature is presented to understand complexities in the sensor design problems and to determine the directions in which research is heading. Several challenges for the design of robust sensor network for large WDNs with less computational efforts are discussed and recommendations are presented.