Distribution system water quality monitoring is still in its infancy, but there is an emerging realization of its potential value. The initial emphasis has been on developing ways to detect deliberate or inadvertent chemical and biological attacks on water distribution systems. The potential for harm was made clear in the case of the Milwaukee Cryptosporidiosis outbreak of 1993 (McGuire, 2006), in which thousands of people became ill. Historically, water quality has been monitored on a limited basis to determine compliance with regulatory standards such as periodic monitoring for chlorine residual and total coliform. Yet the need for more comprehensive monitoring has been demonstrated by a growing body of research that indicates that water quality can potentially change between the water treatment plant (WTP) and the ultimate consumer (Baribeau et al, 2005; Zhang et al, 2002; LeChevallier, 1990). Therefore, in addition to alerting a utility of a contamination event, a second potential application for distribution monitoring is to ensure that the water received by the public has not degraded below acceptable standards. While examining databases from numerous well-operated utilities (Cook et al, 2008), the authors determined that real-time distribution system data can provide early-warning of: nitrification in chloraminated water systems; elevated disinfection byproducts (DBP); explanations for turbidity changes in the distribution system that are unrelated to distribution system operation; and, estimating detention times between the WTP and distal locations. The last finding, which is beyond this paper's scope, could allow near-real-time hydraulic model calibration. This paper presents three cases in which distribution system monitoring data was used to detect early water quality degradation, to offer explanations as to why the degradation occurred, and to propose that such monitoring, along with data analysis, could be used to enhance effective operational decision-making. Includes 16 references, figures.