On Space Station Freedom experiments of months duration will be performed to study the long-term effects of space conditions on living organisms, which will require the measurement of many biochemical parameters at frequent intervals. In addition, monitoring of the water recycling process is essential for assuring the health and well-being of the crew. Returning samples to earth for analysis will be impractical, since the analytical results would become available only after several months due to the infrequent logistics flights of the Space Shuttle. Hence, onboard analysis of a large number of analytes will be essential on Space Station Freedom. Conventional chemical analysis during spaceflight is complicated for several reasons. Sample treatment is more difficult in a weightless environment than on earth. Use of toxic reagents can only be permitted under conditions of double containment. Analytical instruments take up space and use power. The usual bioanalytical procedures are rather time-consuming. The use of chemical sensors offers considerable advantages over conventional analysis onboard spacecraft. A chemical sensor consists of a selector which selectively interacts with the analyte present in a mixture of substances, and a transducer which produces an electric signal in response to the interaction of analyte and selector. The transducer signal thus provides a quantitative and selective measurement of the analyte. Since the signal is usually in digitized form, rapid transmission to the ground is possible. In principle, chemical sensors can provide (near) real time monitoring of many important analytes. In some cases they can even provide continuous monitoring of such analytes. The sensors, and even the ancillory instruments, are small compared to conventional analytical instruments. Their power consumption is low. Sensor measurements do not require extensive sample treatment before analysis. In many cases a sensor can simply be inserted in or be placed on the organism, or be placed in the water flowing through the water recycling system. Sensors thus allow efficient utilization of the scarce resources of crew time, pressurized volume, and power.