This collection of documents covers the relations of the great powers between 1968 and 1975, the dates being fixed by the Soviet invasion of Czechoslovakia and the signing of the Helsinki Agreement on European security respectively. In selecting from the very wide range of printed sources available we have kept purely illustrative material to a minimum and printed only those documents which either indicated a change in the policy of a particular country from its established position or recorded a substantive and agreed change in the structure of great-power relations. The coverage in the various sections of the volume is not in all cases, therefore, bound by the period covered by the collection as a whole: if there was no major structural change after, say, 1973 the continuing diplomacy on that subject will not be documented. Even using these criteria we have had to keep the scope of our selection in line with the realities of publishing costs. Where we have been forced to exclude supporting documents to major agreements or material which might have helped to complete the picture, we have cited the appropriate references in the Introduction or in Editor's Notes which are printed following the documents to which they refer. Otherwise we have attempted to keep footnoting to a minimum: as a general rule, cross-references have been confined to documents which fall within the 1968–75 period and which appear in other sections of the volume.