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 interested in the important and fascinating field of interactions between plants and insects. We were pleased and honoured to organise this symposium, which took place June 24--30, 1995 in Gwatt on the shores of the Lake of Thun in Switzerland. 168 participants from 26 countries from all over the world actively took part in the symposium by contributing 12 key-note lectures and a total of 141 oral presentations and posters. The favourable response and the lively interaction of the participants in all symposium activities is the clearest indication of the success of SIP-9. The organisers appreciated the enthusiasm and the willingness to collaborate shown by all participants. The following volume contains written contributions (72) of only half of all presentations. This is due to the fact that we decided to produce not only an account of the proceedings but also to publish all contributions as a special volume of the journal Entomologia Experimentalis et Applicata. This procedure was last adopted in 1978 for SIP-4, organised by Reginald F. Chapman and Elizabeth A. Bernays, and ensures a wide distribution of the papers within the scientific community and easy access through libraries. Inevitably we had to employ the same review procedure as applicable for the manuscripts regularly submitted to Entomologia.