Industrial Crystallization Symposia have been organized by the Crystallization Research Group at the Czechoslovak Research Institute for Inorganic Chemistry, Usti nad Labem, since 1960. Over the years, the increasing popularity of the unit operation of crystallization has been clearly demonstrated by the steady increase in numbers of both the papers presented and the attendances at the meetings. The 6th Symposium (1-3 September 1975) was organized jointly with the European Federation of Chemical Engineering Working Party on Crystallization, and the 44 papers presented were arranged into four sessions - A: Secondary Nucleation, B: Crystal Growth Kinetics, C: Crystal Habit Modification, D: Crystallizer Design, E: Indus­ trial Crystallizer Operation and Case Studies. The same groupings are preserved in this edited version of the proceedings. This is the first time that the Industrial Crystallization Symposium papers have appeared in one volume. After the 5th (1972) Symposium, authors we.re encouraged to submit their papers to an international journal specializing in crystallization. However, the results were not altogether satisfactory in that less than one third of the papers presented at the meeting were offered for consideration. This time, therefore, the organizing committee decided to attempt to keep the papers together by making arrangements for their pUblication by Plenum Press.