Electric or hybrid vehicles represent moving silent objects in potentially complex environments. As such, they definitely can be considered either as a blessing, with regards to noise pollution, or a curse, with regards to user-experience. In an ecological analogy – and all things considered –, the advent of this new type of mobility artefacts can be compared to the introduction of a new species in a given ecosystem that should find its right place by means of external signs embodying its reality with regards to others. On the basis of this approach, and because these quiet vehicles (electric, hybrid or even recent quiet internal combustion engine cars) mostly tend to exist in noisy and heterogeneous urban environments, it seems inevitable to tackle issues related to their silence. This leading property (quietness) should be studied, designed and formalized in order to provide relevant and efficient answers to the main problems they address, at least, for the next few decades (before the other noisier species finally disappear …). Within this scope, the paper will present a synthesis of the topic: firstly, by setting out the context of use (silent vehicles and accidentologic situations); secondly, by inventorying several proposed solutions (driver or pedestrian-centred); thirdly, by argumenting the sound design approach and presenting some prototyped or industrialized solutions; and fourthly, by giving the basis and the recent developments on normative works that are undertaken at national or international levels.