Categorizing organisms as either prokaryotes or eukaryotes was ?rst int- duced in 1937 and proposed again in 1957 (Beck 2000); however, it was some years later, prompted to a certain degree by a publication by Stanier and van Niel(1962),beforetheconceptbecamemorewidelyaccepted. Atthisearlystage inthelate1950s,ourcompleteunderstandingoftheultrastructureofbothcell types,but especially prokaryotes,was still initsinfancy and awaited the more common and extensive use of electron microscopy as well as a multitude of other techniques and instrumentation. Prokaryotes were generally viewed as primitive simple cells with littleintracellular structure. The existence of some storagebodies had been demonstrated (see Vol. 1 of this series), but littleelse was known about the internal organization (?rst chapter, this volume). The absence ofmitochondriaorchloroplastsappeared certain, but thestateofthe nucleic acid within the prokaryotic cell and the presence/absence of mitosis were still somewhat controversial subjects. That our concept of a prokaryote has undergone a remarkable transformation during the last 50 years should becomereadilyapparent asoneproceedsthroughthechaptersinthisvolume. Furthermore, the reader should come to realize that the rigidclassi?cation of anorganism aseither aprokaryoteoreukaryoteisnot always soclear-cut. We are quite sure that the next 50 years will provide a clearer understanding of the complex intracellular structures presently known as well as bring to light surprising new ones. Aswegatheredinformationontheintracellularstructuresfoundinase- ingly endless variety of prokaryotes it became obvious to us that the c- ponents were so numerous that it would be impossible to include topics for all of them in one “reasonably sized” volume of Microbiology Monographs. Afterconsideringdifferentalternatives,wedecidedtoplacethosecomponents with primarily a metabolic storage/reserve function in Vol.