Among the microorganisms important in radiation processing of foods a major distinction is made between bacterial spores which are highly radiation and heat resistant, and vegetative cells which are generally radiation and heat sensitive. In food processing, this is a traditional distinction, although one should be aware of the many exceptions to this rule in radiation microbiology. Occasional reports describe highly radiation sensitive spores or vice versa highly resistant vegetative cells. Other organisms investigated as potential spoilage agents and public health risks include organisms of extremely high radiation resistance, particularly Micrococcusradiodurans, M. radiophilus, Moraxella spp., and Streptococcus faecium, as well as molds capable of production of mycotoxins. Bacterial spores are also dormant structures with little or no free water in their cytoplasm. In distinction to viruses, however, the spores have within their core all the essential cell enzymes and cell components needed to initiate growth and to develop independent metabolism.