This chapter attempts to bring to light the field of one of the less popular branches of the open source software family, which is the open source database management systems branch. In view of the objective, the background of these systems will first be briefly described followed by presentation of a fair generic database model. Subsequently and in order to present these systems under all their possible features, the main system representatives of both open source and commercial origins will be compared in relation to this model, and evaluated appropriately. By adopting such an approach, the chapter’s initial concern is to ensure that the nature of database management systems in general can be apprehended. The overall orientation leads to an understanding that the gap between open and closed source database management systems has been significantly narrowed, thus demystifying the respective commercial products.