International audience; This chapter investigates the process through which radical change is governed. While previous work has mostly focused on emergence, we focus on initial diffusion and the conditions under which potential breakthrough innovations can get out of the ‘protected spaces in which they have been tested. We are thus interested in the collective efforts that are developed to ‘shape markets’ and to create, following Fligstein, relevant ‘market infrastructures’, that is the set of rules (what actors are allowed to do), of norms (what they ought to do) and of values (what they want to do). We follow analysts on the central role of arenas as the settings in which “individual and collective actors interact to define the cognitive and normative dimensions of a problem”. But we show, through the example of nanotechnology, that any new breakthrough technology drives to the emergence of multiple arenas proposing each their approaches and tools for governing the new technology. Studying for nanotechnology their internal dynamics, the articulations and alignments between arenas that have taken place, we analyse the conditions of ‘success’ of arenas. Successful arenas as those than manage to enrol new actors, enlarge their initial remit while seeing their ‘outputs’ taken over by other arenas. Four aspects matter for the effective success of an arena - all linked to legitimacy: the degree of specificity, the degree of openness, the level of transparency and the degree of structuration. This drives us to propose the notion of governance arrangement to characterise the specific alignment between arenas and the robust compromise that enables the stabilisation of market infrastructures. Until the governance arrangement is set, existing uncertainties (technical or social) do not allow actors to move forward in the development of innovations and markets are not structured because the market infrastructures have not been agreed upon.