To determine the microbiological quality of water for potable and other purposes, there is a need for rapid methods to enumerate viable bacteria. This is of particular importance for the proposed water recovery systems planned for the United States Space Station, in which wastewaters including hygiene water and urine will be reclaimed for potable use. Existing microbiological culture methods are limited by the time taken to obtain results and because it is not possible to detect the total microbial populations by these methods. We have been investigating direct microscopic methods which detect individual bacterial cells. Fluorogenic compounds are used which are taken up by active cells, permitting a direct assessment of physiological activity. The methods are being adapted for use with membrane filtration which permits concentration of small numbers of cells from large volumes of water. Procedures for direct examination of cells growing on surfaces as biofilms have also been devised. Stained samples are examined by epifluorescent microscopy, a tedious technique which can be automated using computerized image analysis systems. The results of these methods are obtained within a few hours, compared to the 2-7 days required for total viable bacterial counts using traditional agar plating methods. Some rapid methods are reviewed, and rapid techniques for physiological assessment compared with traditional methods. Prospects for developing rapid methods to detect specific bacteria, combined with physiological assessment, are discussed.