ThisvolumeconsistsoftherefereedpaperspresentedattheFifthInternational ConferenceonLogicProgrammingandNonmonotonicReasoning(LPNMR 99) heldatElPaso, Texas, inDecember1999. LPNMR 99isthe fthinaseriesofinternationalmeetingsonlogicprogramming andnonmonotonicreasoning. FourpreviousmeetingswereheldinWashington, U. S. A., in1991, inLisbon, Portugal, in1993, inLexington, U. S. A., in1995, and inDagstuhl, Germany, in1997. TheaimoftheLPNMRconferencesisto- cilitateinteractionsbetweenresearchersinterestedinlogicbasedprogramming languagesanddatabasesystemsandresearcherswhoworkintheareasofk-...