In September 2000, the University of Bayreuth, Germany, hosted the Fourth International Meeting on Thermodi?usion (IMT4). TheIMTconferenceswerebornfromtheideaofbringingtogetherresearchers in the ?eld of thermodi?usion. Under the auspices of the European Group of Research in Thermodi?usion(EGRT)theconferenceseriesstartedin1994with IMT1 in Toulouse and has been continued every other year with IMT2 (Pau, 1996), IMT3 (Mons, 1998), and IMT4 (Bayreuth, 2000). The next conference, IMT5, will be held in 2002 in Lyngby, Denmark. Thermodi?usion,alsocalledthermaldi?usionortheLudwig-Sorete?ect,- scribes the coupling between a temperature gradient and a resulting mass ?ux. Although the e?ect was already discovered in the 19th century by Ludwig and Soret, it has gained growing interest during the last years due to improved - perimentaltechniqueslikestate-of-the-artthermogravitationalcolumns,modern opticalmethods,?owchannels,andmicrogravityexperiments,tomentiononlya few. We are still far from a detailed microscopic picture, but analytical theories have been improved and the availability of fast computers and e?cient al- rithmsfornonequilibriummoleculardynamicssimulationshasprovidedvaluable input from the theoretical side. TheIMTconferencescoverallaspectsofthermodi?usionfromfundamentals to new applications. Traditionally, the focus has been on the ?uid state, ra- ing from mixtures of simple liquids to more complex systems such as critical mixtures, electrolytes, polymers, colloidal dispersions, or magnetic ?uids. IMT4 tried to widen the scope by including a plenary lecture about thermodi?usion in ionic solids. Scienti?c input comes from diverse disciplines such as physics, chemistry, engineering, and geophysics. Sadly, Leo Kempers passed away while this book was being prepared. Many ofushavelostafriendandrespectedcolleague.Hismanuscripthasbeenbrought into its ?nal state by A. Shapiro, whom we want to thank here.