Climate change has become in 20 years one of the greatest economic, environmental and social challenges of our modern society. A wide variety of organizations – NGOs, governments, business, international bodies, local communities, research think tanks – are working together to design and implement a low carbon society. In this particularly uncertain context, characterized by distributed, lacunar, messy and sometimes contradictory scientific knowledge, the actors fail to converge on a common project regarding the architecture of a low carbon society. Projects and visions vary among actors and over time. Nevertheless, it is commonly admitted among experts and economists that a carbon price that would be stable, predictable and fair could provide the long term coordination that is needed to drive the implementation of a low carbon society. “A price of carbon would solve any problem” said a French expert in a recent interview . A ‘right’ price of carbon would diffuse in the economy and provide long term drive for technology breakthroughs and switch to low carbon products said another one . Such a ‘right’ carbon price would then stir up the profound societal changes that are required. In Europe, these great expectations over a ‘right’ carbon price have aroused an on-going design activity that enables the existence of the European carbon market (EU-ETS). The ‘official’ story of how carbon markets were designed and implemented, as it is told in economic handbooks and in the press, is well known and widely documented (e.g. Braun, 2009; Ellerman & al, 2010; Hourcade, 2002; Cass, 2005; Wetestad, 2005). According to this story, environmental economics is supposed to be particularly performative as it presents carbon markets as the output of thirty years of research program in environmental economics initiated in 1960 by Ronald Coase and his famous article, ‘the problem of social costs’. This common representation tends to overlook three activities that enabled the concrete performation of theoretical economy; that is to say design, negotiation and revision. We propose to adopt the perspective of (Callon 2009): “How are the different knowledge and know-how transported, experience capitalized on, and evaluations conducted?” We claim in this paper that in the case of carbon markets, the existence of design spaces that mediate between economics and economy (Guala, 2007) is central to explain the performation of the EU-ETS.