The reason why insect-plant relationships attract more and more attention has been given by Professor Schoonhoven in the introductory lecture to this symposium: `The green blanket on the earth's surface with its myriads of insects has already revealed some of its complex workings, but contains many more facets which need to be discovered to gratify our insatiable curiosity, as well as help to improve agricultural production'. The 9th International Symposium on Insect-Plant Relationships (SIP-9) was once more, following the tradition established in 1958, a forum for investigators in both basic and applied entomology. Participants from 26 countries throughout the world presented 12 keynote lectures and a total of 141 oral presentations and posters. The proceedings volume contains 72 contributions of both oral lectures and posters which were all peer-reviewed by two independent referees.