PrefaceThis volume of the Annals of Operations Research, contains a collection of papers published in memory of Peter L. Hammer (1936Hammer ( -2006.As we recall further down, Peter made substantial contributions to several areas of operations research and discrete mathematics, including, in particular, mathematical programming (linear and quadratic 0-1 programming, pseudo-Boolean optimization, knapsack problems, etc.), combinatorial optimization (transportation problems, network flows, MAXSAT, simple plant location, etc.), graph theory (special classes of graphs, stability problems, and their applications), data mining and classification (Logical Analysis of Data), and, last but not least, Boolean theory (satisfiability, duality, Horn functions, threshold functions, and their applications).The Call for Papers invited authors to submit research papers in all of these areas. We were very happy to receive a large number of high quality submissions along the lines explored by Peter, and eventually we selected 22 original contributions to appear in this special E. Boros ( ) RUTCOR,