There is nowadays such a wealth of publications in hydrology, that it seems unlikely that any hydrologist can find the time to read even a decent part of it. Therefore, we have planned this preface as a rapid introduction to the papers that constitute this volume. Our aim is to convince you, the reader, to keep reading in order to discover the original contributions, which we believe are really worth the time you will spend on them. This volume contains 25 papers, many of which follow on from the presentations made at the last two MOPEX workshops held in July 2004 at ENGREF, Paris, France and April 2005 at the IAHS meeting in Foz do Iguacu, Brazil. One of the objectives of this volume is to show how valuable it is to work on large data sets in hydrological modelling. Articles are organised into five sections: The first section provides an introduction into the goals of the MOPEX project, and presents the databases that were used by the participants, a part of which is made available to the hydrological community on the CD-ROM (DVD?) accompanying this volume. The second section groups four review papers, which were not presented during the MOPEX workshops, and which have been solicited specially for this volume, in order to provide alternative views on the use of large sample basin experiments in hydrology. The third and the fourth sections present model parameterisation experiments based on samples of a large number of basins: in the third section, the focus is on the databases that were gathered specifically for the MOPEX program, while the fourth section presents regionalisation and parameterisation studies based on other large hydrometeorological databases. Lastly, the fifth section presents a compilation of the most recent results of the project, and discusses its perspectives.