One of the most important cellular events in all organisms is protein synthesis (translation), which is catalyzed by ribosomes. The regulation of translational activity is dependent on the environmental situation of the cell. A decrease in overall translation under stress conditions is mainly accompanied by the formation of functionally inactive 100S ribosomes in bacteria. The 100S ribosome is a dimer of two 70S ribosomes that is formed through interactions between their 30S subunits. Two mechanisms of 100S ribosome formation are known: one involving ribosome modulation factor (RMF) and short hibernation promoting factor (HPF) in a part of Gammaproteobacteria including Escherichia coli, and the other involving only long HPF in the majority of bacteria. The expression of RMF is regulated by ppGpp and cyclic AMP-cAMP receptor protein (cAMP-CRP) induced by amino acid starvation and glucose depletion, respectively. When stress conditions are removed, the 100S ribosome immediately dissociates into the active 70S ribosomes by releasing RMF. The stage in the ribosome cycle at which the ribosome loses translational activity is referred to as 'Hibernation'. The lifetime of cells that cannot form 100S ribosomes by deletion of the rmf gene is shorter than that of parental cells under stress conditions in E. coli. This fact indicates that the interconversion system between active 70S ribosomes and inactive 100S ribosomes is an important survival strategy for bacteria.