At this stage these results are hardly generalized since they are strongly contextdependent. Above all, these studies address the effectiveness of GPP through the potential gains in absolute terms. Following the seminal papers by Marron (1997, 2003), Lundberg and Marklund (2011, 2012) are the first ones adopting a welfare point of view, comparing GPP to other environmental policy instruments given a benchmark such as an efficiency perspective. They show that public procurement is not necessarily contributing to achievement of environmental targets at least cost to society. Lundberg and Marklund (2012) highlight the fact that introducing green criteria in the award procedure (through the scoring rule) increases complexity and reduces transparency. As a consequence, the environmental gains achieved by GPP may be at the expense of a reduction of the gains achieved by the procurement auction per se, such as upholding competition. In doing so they theoretically prove that GPP is not a cost-effective environmental policy instrument unless firms are homogeneous (which is unrealistic) or public buyers are able to formulate call for tenders and scoring rule that are “technique neutral” in the sense that they function as a tax and do not regulated input and process choices of the firm. They also confirm Marron’s (2003) conclusion that a per-unit tax on emissions à la Baumol and Oates (1971, 1988) leads instead to cost-effective reduction. Another way to implement GPP is through the use of environmental performance clauses in the contracts. This is notably the case for PPPs and more particularly for the Private Finance Initiatives (PFI) contracts for which the payment is based on making the infrastructure available and is usually affected by the capabilities of the operator to meet performance targets. Merck et al. (2012) analyzed how cities use PPPs to achieve their green infrastructure objectives covering a wide diversity of sectors (public transport, energy, water, buildings, waste, urban development) and contractual forms. In brownfield projects, the private sector participates (as investors and operators) in work on or refurbishment of existing infrastructure facilities; greenfield projects develop new infrastructure, such as a new wastewater treatment facility, a stadium or a bicycle sharing system. While both greenfield and brownfield PPPs can have environmentally friendly characteristics, it is easier to make greenfield PPPs truly green through the inclusion of environmental criteria in the tendering procedure. One of the most innovative green PPPs is Energy Performance Contracting (EPC).4 The objective of EPC is not the execution of works (supply of goods or services), but the improvement of energy efficiency (i.e. reduction of energy consumption). This is an innovative approach to contract design and thus raises specific issues related to performance measures and verifiability under the context of legal and technological uncertainty. The use of a performanceoriented contract is only possible when energy efficiency is perfectly measurable, with observable and verifiable indicators. Depending on the sector and the objectives of a given project, such a measure may be difficult to establish and contest. The more difficult the control, the more likely ex post conflicts concerning efficiency targets, observed performances and responsibilities will occur. These conflicts are costly and affect the efficiency of PFI.