This book offers an in-depth analysis of the cell biology of cyanobacteria, a group of phototrophic microorganisms performing an important function in the biosphere. The chapters present the author’s and her colleagues’ pioneering investigations of the ultrastructure of cyanobacteria under high-light and dark conditions, during irradiation by extremely high fluxes of light, in the course of L-transformation and within model associations and natural symbioses with plants. Diverse patterns of ultrastructural change are illustrated in electron micrographs and schematics. The book further introduces a new concept of “bacterial ultrastructural plasticity” - the reversible rearrangement of ultrastructure in response to environmental changes, as a strategy for finding and investigating cell adaptation mechanisms and intraspecies structural diversity of cyanobacteria and other prokaryotes.