This volume covers the use of modern solvent systems for the performance of more efficient, selective and economical organic transformations. During the last twenty years water has emerged as a new useful reaction media for the performance of organic or metal catalyzed reactions. More recently, other reaction media like perfluorinated solvents or supercritical CO2 have proven their utility for many organic synthesis as well as in combinatorial chemistry. Also the most representative industrial applications of some of these modern solvent systems are described and finally the synthetic possibilites of performing organic reactions in the absence of solvents are examined.