Metals in the hydrological cycle represent a very broad subject covering all parts of the geological cycle. The present version of this book, therefore, would not have been possible without the comments and suggestions for improvement on draft ver­ sions of the various chapters by a large number of colleagues. We wish to express our gratitude to: P.A. Cawse (AERE, UK), J.N. Galloway (University of Virginia, USA) and S.E. Lindberg (Oak Ridge National Labo­ ratory, USA) for reviewing the chapter on atmospheric trace metals. G. Batley (CSIRO, Australia) and B.T. Hart (Chisholm In­ stitute of Technology, Australia) for reviewing the chapter on speciation of dissolved metals. E.K. Duursma (Delta Institute, The Netherlands), J.M. Bewers and P.H. Yeats (Bedford Institute of Oceanography, Canada) and D. Eisma (Netherlands Institute for Sea Re­ search, the Netherlands) for reviewing the chapter on estuaries. P. Baccini (EAWAG, Switzerland) and W. Davison (Fresh­ water Biological Association, UK) for reviewing the chapter on lakes. E.T. Degens (University of Hamburg, W-Germany) for re­ viewing the chapter on the oceans, and J.P. Al (Public Works Department, The Netherlands) for reviewing most of the indi­ vidual chapters. Without the collaboration of these colleagues this book would not have been possible in its present form.