The gas vacuole was first observed in 1895, but details of this structure (gas vesicles) aswell as discovery of the other structures covered in this monograph (proteasomes, phycobilisomes,chlorosomes, carboxysomes and carboxysome-like inclusions, magnetosomes, intracytoplasmic membranes,membrane-bounded nucleoids, pirellulosomes, anammoxosomes and the cytoarchitecture of Epulopiscium spp.)awaited the availability of the transmission electron microscope and related technologies. Additionaladvancements in electron microscopy were required for the optimal visualization of some structures.