Water is an essential molecule for all life on earth and it carries numerous other species including different elements and neutral molecules which necessarily decide its usability for a variety of applications. The presence of external species in water occurs either naturally or due to anthropogenic activities. Though some of them are essential nutrients for living organisms, many of them might lead to adverse health effects. Rapid industrialization and the ever-increasing human population have resulted in an increased demand for potable water. Unfortunately, the scarcity of quality potable water owing to the depletion of resources in several countries demands the identification of new resources and day-to-day water quality monitoring. Indeed, an inadvertent mix of sewage and domestic wastewater in drinking water resources also contributes to water pollution, in addition to industrial waste which is a predominant water-polluting activity. Thus, continuous water quality monitoring at the source is essential which could be achieved by the systematic analysis of the contaminants present therein, even in remote places. While the traditional analytical techniques are quite expensive and hence cannot be deployed in remote places, molecule or material-based identification, that is, sensing of specific target species has been found to be the cost-effective method. Unfortunately, most of the probes that have been developed were specific to one target, which becomes a drawback when analysing the actual sample that would probably consist of several toxic species. Thus, simultaneous analysis of different toxic species with distinctly different signalling pathways is required as a practically viable solution. This chapter will focus on the importance of water quality monitoring, methods, and sensors that operate on colorimetric, luminescent, and electrochemical principles. The toxic species present in the water will be restricted to the metal ions. Furthermore, the molecules which sense more than one or two toxic species simultaneously will be incorporated and the reports of the integrated sensor arrays will also be included.