Page  549 ï~~Space at Performer's Fingertips Phillippe Menard This paper is devoted to space in relationship first with traditional analog technology when the electroacoustic composer spatializes his music, at the end of the musical "assembly line"; secondly, with the digital technology which has given the composer the opportunity to work in empty space at the beginning of the musical process, through instrumental gestures in a network of electronic sensors. It echoes Xavier Chabot's paper issued in a Computer Music Journal, Vol. 14, No. 2. Through my own experience with SYNCHOROS, an interactive live performance system of my own design, I discuss, as a musicologist could do, how both composition and performance have changed with interactivity, the composer becoming a "meta-creator' of systems of potential musics, the performer becoming a co-creator of only one of them, actualized by his live choreographic performance, monitored by electronic sensors (phototransistors, infra-red sonars and others). Looking for what is really new, I come to isolate three major concepts in some new interactive instruments: immaterially (free hands or free body in free space); multiplicity instead of bi-univocly (one gesture can generate various programmable responses); and interactivity (gestures are "dialoguing" with instruments capable of structured musical proposals). I explain how I have until now experimented with SYNCHOROS in three different contexts: in a "soloist" configuration of my own performances; in a "chorist" configuration for a theater play, one actress controlling the music; and finally in an "agorist" (crowd) configuration for a museum installation, the visitors controlling lights and sounds. Discussing instrument design, I examine some differences between what I call "parametric" instruments (old or new) and "structural" or informational instruments, emphasizing the qualities of the latter. I drop a few words on the developing software environments, that is first HMSL followed by MAX. I explain how the composer/performer can now play the whole studio on stage; and how the main goal in performing the new interactive instruments stays a real and interactive communication with the public. In the last part, I more deeply discuss interactivity, relating it to the improvisatory part in both creation and performance, my basic assumption being that "the closer the instrumental gesture is to the raw matter, the more interactive the creation is." Taking improvized music as an example, I show why human dialog has become "the" model for interactive creation. After a long period of mediatization, I feel interactivity is the revenge of the "immediate" against the "mediate". ICMC 549