Risk assessment is one part of a comprehensive risk management process, known as risk analysis. The risk characterization step within microbial risk assessment (MRA) serves as the bridge between risk assessment and risk management. Although a variety of MRA approaches are in use, limited analyses of MRA frameworks or their underlying principles and concepts exist. Organizations' desires to develop a unified MRA approach have been tempered by the realization that flexibility is essential for addressing various legislated mandates and regulations and for meeting diverse field application needs. This paper was developed to help EPA's Risk Assessment Forum, Microbial Risk Assessment Working Group to obtain new knowledge and insights about the nature and characteristics of available MRA frameworks and applications. This report includes a review of recent MRA policies and guidelines and a comparative analysis of 13 MRAs conducted or sponsored by governments worldwide. Two forms of risk assessment-chemical risk assessment and ecological risk assessment-provide the foundations for MRA as it was practiced at the time of this review. The National Academy's widely used four-step risk assessment paradigm is the prevailing context from which many modelers have approached microbial risk assessment. The most commonly cited underlying principles for MRA include: make public health protection the priority, base MRA on sound science, ensure transparency, use a structured and consistent approach, and allow for iterations. The dynamic aspects of pathogens, environmental processes, human populations, and the interrelationships of these entities are increasingly being noted as important MRA modeling issues. Four fundamental types of MRA frameworks were found: chemical risk assessment modified chemical risk assessment, problem definition followed by chemical risk assessment, and the International Life Sciences Institute (ILSI) framework. No agency has applied the ILSI framework in a complete MRA. Recommendations are made for advancing MRA through systems methodologies and communication strategies.