The ambiances of tomorrow are explored through their design by architects and through their activation by the individual perception and sensibility of future users. If the ambiance is an overall and diffused feeling then it needs a random event in order to awaken the perception of its quality. Based on the analysis of 25 different projects of public spaces built recently in Europe (France, Greece, Sweden and Spain), the paper shows that the creation of ambiances in architecture depends on the dialectical relation between the back-plan and the unfolding events, which oscillates between conflict, negotiation and sympathy.