This volume contains papers selected from those presented at the International Symposium on the Analysis of Hydrocarbons and Halogenated Hydrocarbons in the Aquatic Environment, May 23-25, 1978. The Symposium was organized by the National Water Research Institute of Environment Canada and the Institute for Environmental Studies of the University of Toronto. The purpose of the Symposium and of this volume was to bring together information on the analyses, behaviour and effects of hydro carbons and halogenated hydrocarbons on the aquatic environment. This class of contaminants presents many difficult analytical pro blems, and to a large extent our ability to identify environmental problems and assess their severity, depends on the availability of proved analytical techniques. By exposing workers in this field to the techniques and results of others we hope that progress can be made towards solving the many problems caused by these substances. The papers in this volume are divided into five categories: two plenary session addresses given by J.P. Bruce and O. Hutzinger, 11 papers describing quantitative analytical results, 12 papers on analytical methods, 9 papers on incidence, monitoring and pathways; and finally, 13 papers on drinking water, health and biological effects. We were fortunate in having the support of the many organizations and individuals listed after this preface. In particular we thank B.F. Scott and R. Knechtel for their help in organizing the Symposium, S. Paterson, C. Straka, B. Reuber, A. Bobra, R. Wan, S. Austin, G.E.