The precise mechanisms of embryo implantation remain unclear: the uterus is a "hostile environment" which should become "neutral" at the time of implantation. Advances have been made both in terms of embryo quality and in implantation techniques, making better knowledge of the uterus-embryo dialogue essential. To meet this challenge, the Pharmaceutical firms Serono France and ARCEFAR have organized a symposium entitled "Embryo implantation: basic principles and practical aspects". The symposium, presided by André Hazout and Yves Menezo, will first focus on the fundamental role of the ovum then will explore the complex immunological processes involved in the uterus-embryo dialogue.