DLV is a deductive database system, based on disjunctive logic programming, which offers front-ends to several advanced KR formalisms. It has been conceived by an Italian-Austrian research team (of the University of Calabria and the Vienna University of Technology). The system supports a language based on a logical formalisms with a very high expressive power so that programs are able to represent relevant practical problems in presence of incomplete or contradictory knowledge. The first release of the system became available in 1997, after several years of theoretical research. It has been significantly improved over and over in the last years, incorporating new features and relevant optimisation techniques in all modules of the engine. Nowadays it represents the state of the art among Knowledge Representation and Reasoning (KRR) systems. Thanks to a long lasting theoretical and implementation effort, the language is now supported by an efficient run-time system that exploits techniques developed throughout the years by the scientific and industrial database community. By analysing the nature of its specific input, the system is able to apply the techniques that better reflect the complexity of the problem at hand, so that “easy” problems are solved fast, while only harder problems involve methods of higher computational cost.