The environment of Space Station Freedom will present unique challenges with respect to microbiological monitoring. Recycling of air and water will provide potential reservoirs and transmission routes for microorganisms. Since S.S. Freedom will operate as a marginally closed environmental system, sources of potential microbial hazard must be rapidly identified in order to minimize risk to the crew and structural or operational systems.Our recent efforts have been directed toward the identification of relevant concepts and methodologies for microbiological monitoring in Space and to define the requirements of a near real-time microbiological monitor. Initially, five concepts applicable to microbiological monitoring were defined. Within these concepts, twenty-eight methodologies were identified which have the potential for microbial detection and/or identification. Evaluation criteria were developed and all identified methodologies were evaluated. Of these, only one-third met the minimum requirements that were established for a near real-time microbiological monitor. Through further technical evaluations, the five candidates yielding the highest scores were chosen for engineering and feasibility trades. The candidate methodologies include: laser light scattering, primary fluorescence, secondary fluorescence, volatile product detection and electronic particle detection. In addition, the advantages and disadvantages of the candidate method are described.