Megaplasmids in Archaea -- Haloarchaeal Megaplasmids -- Megaplasmids and Biodegradation -- Megaplasmids and the Degradation of Aromatic Compounds by Soil Bacteria -- Catabolic Plasmids Involved in the Degradation of Polycyclic Aromatic Hydrocarbons and Heteroaromatic Compounds -- Megaplasmids in Plant Symbionts and Pathogens -- Sinorhizobium meliloti Megaplasmids and Symbiosis in S. meliloti -- The Megaplasmid pNGR234a of Rhizobium sp. Strain NGR234 -- Ti and Ri Plasmids -- Megaplasmids in Human Pathogens -- The Virulence Plasmids of Shigella flexneri -- The Three Bears and Virulence-Associated Plasmids in the Genus Yersinia -- Virulence Megaplasmids in Bacillus anthracis and Their Relatives in the Bacillus cereus Group -- Megaplasmids and Special Metabolic Functions -- Megaplasmids in Cupriavidus Genus and Metal Resistance -- Megaplasmids of Aerobic Hydrogenotrophic and Carboxidotrophic Bacteria -- The Megaplasmid pAO1 of Arthrobacter Nicotinovorans and Nicotine Catabolism -- The pMUM Megaplasmid of Mycobacterium ulcerans and Closely Related Mycobacteria: A Blueprint for the Synthesis of Mycolactones -- Megaplasmid pKB1 of the Rubber-Degrading Bacterium Gordonia westfalica Strain Kb1 -- Megaplasmid pMP118 of Lactobacillus salivarius. Megaplasmids are extrachromosomal genetic elements in the size range of 100 kb and larger. They are found in physiologically and phylogenetically diverse groups of bacteria and archaea. By definition, megaplasmids are not essential for the viability of their hosts under all growth conditions, but paradoxically many megaplasmids carry the genetic information for the defining and characteristic traits of the organism in which they reside. Microbial Megaplasmids reviews our knowledge of the extensively studied representatives, such as the catabolic plasmids of the pseudomonads, the rhizobial Sym plasmids, the Ti plasmids of the genus Agrobacterium and the giant enterobacterial virulence plasmids. It also presents snapshots of more recently discovered megaplasmids. The contribution of megaplasmids to the biology of their hosts is described, highlighting the interactions between megaplasmid and chromosomal genes.