It is our great pleasure to introduce the Proceedings of the Twenty-First European Peptide Symposium, held in Platja d' Aro, Spain, September 2-8, 1990. Over seven hundred scientists from nearly thirty countries, mostly European but also from the Americas, Australia and Japan, assembled at the largest-to-date in a series of European Peptide Symposia to present and discuss their recent findings in the field of peptides. Whenever scientists meet, a good deal of their interaction cannot be appropri ately recorded in book form. Fruitful early morning dialogues go regrettably unrecorded or, at most, precariously scribbled on ephemeral breakfast napkins. Many a brilliant late night debate is bound to fade away through the mists of a noisy discotheque. Alas, this book will unfortunately ignore these potentially splendid contributions to science! No effort has been made either to register the discussions that followed the oral presentations of the Symposium, or the two open sessions on special topics of general interest. Spontaneity, the main appeal of oral discussion, is somewhat lost, we believe, on literal transcription. Once all these non-recordable forms of scientific communication are discounted one is left with the core of the Symposium, i.e., its scientific communications, both in oral and poster form, and it is to these that the book you have in your hands is devoted.