The genus Pseudomonas was described in 1894 and is one of the most diverse and ubiquitous bacterial genera whose species have been isolated worldwide in all kinds of environments, from the Antarctica to the Tropics, present in sediments, clinical samples, plant, fungi and diseased animal specimens, water, soil, plant rhizosphere, sea, deserts, etc. The taxonomy of the genus has been controversial for years since a lot of bacterial taxa initially included in genus Pseudomonas have been reclassified in other genera or species from a different class of Proteobacteria over the years, as techniques for characterization and classification of microorganisms improved, aiming to set a phylogenetic classification of the species. In this review, the historical evolution of the taxonomy of Pseudomonas is described, and the currently valid criteria and future challenges for taxonomy of the genus and techniques used to achieve the necessary characterization for classifying the species are discussed. Finally, all the validly published Pseudomonas species at present are listed with an overview of their diversity and ecology.