Microorganism contamination is a permanent concern in a wide range of fields, including the water-treatment, food and pharmaceutical industries, in which fast detection is critical to prevent microbial outbreaks. In water monitoring, current procedures for water-quality analysis are based on periodic sampling and detection by culture methods, which are slow, requiring 24–48 h for completion, so that, when first results reach the decision-takers and trigger an alarm, significant time has already passed and the population may have been exposed to a health hazard. There is a need for rapid, reliable detection of contaminants in a broad spectrum of water-management situations. For real-time detection, on-line monitoring seems to be the ideal approach, but the need to adjust the available techniques to autonomous operation and the optimization of response time are substantial challenges. This review presents the findings of an identification study about the state-of-the-art of technologies and commercial devices for on-line biomonitoring of water quality, specifically for the detection of fecal contamination. We also include studies dealing with verification or use of these devices.