As an introduction for several special issues of this journal regarding research on injured microorganisms (IM), the definition of IM and a brief history, current state and significance, prospects and future problems of IM are overviewed. Although an injured cell is generally considered to be sublethally injured, a model including “semilethal” injuries is also proposed here. In the fields of microorganism preservation and food processing, the occurrence of IM has been recognized as a key factor, and the presence of IM has been one of the critical problems in the detection and enumeration of microorganisms. Studies on IM have recently been activated by increasing the recognition of their significance in sterilization, disinfection and viability measurements. The execution of the so-called “the IM project” supported by the Ministry of Agriculture, Forestry and Fisheries, Japan, as well as the establishment of “the Japanese Research Society for Injured Microbes” may further raise awareness of IM. Nevertheless, there are still unknown and nondescript research areas in the IM world. Eight possible problems in the study of IM are presented and the prospects for resolution of these issues are described.