## 2 Background to GPP, SPP and PPI Approaches  
This section reviews the concepts of GPP, SPP and PPI and their contribution to innovation for sustainability, while discussing some challenges for tools supporting public authorities in the institutionalization of GPP, SPP and PPI practices. Green Public Procurement (GPP) is based on the use of environmental criteria in public tendering [17], developing capacity for green supplies and markets [21] and stimulating the innovation capabilities of suppliers [1; 5]. It aims to achieve value for money, while reducing the environmental impact of purchased products and services over their whole life-cycle [21]. GPP is becoming a cornerstone of environmental policies, both at the European Union and Member State level, as well as worldwide [22; 23]. Its role in supporting sustainable consumption and production patterns has strongly increased and currently it is spreading throughout public authorities [1]. By operating as a market trigger, GPP can act as a strong stimulus for eco-innovation [1]. Sustainable Public Procurement (SPP) is the procurement of goods and services incorporating environmental, economic and social concerns into tendering and, therefore, into the procurement process [24; 25]. The approach is therefore very similar to GPP, adding social concerns. According to [PERSON] et al. [17], SPP supports sustainable product-service system innovation. We adopt the definition of Public Procurement of Innovation (PPI), developed by [PERSON] [3], which defines PPI as the whole range of purchasing activities, carried out by public agencies that lead to innovation. This view is in line with [PERSON] [26] and [PERSON] and [PERSON] [27], who also proposed a wider scope for the innovation processes, including: product and service innovation through innovation in organizational processes; societal innovation; the recombination of existing goods and services; and, innovation in the delivery of existing services. In the context of our research we are particularly concerned with PPI for ecological and sustainable innovation. PPI is understood as a tool for stimulating the development of new products--goods, services or systems but it can also refer to public procurement that attempts to open up innovation possibilities, without necessarily targeting new products [9; 27; 28]. [PERSON] et al. [28] recognize that PPI induces radical, new-to-the world breakthrough technologies; promotes incremental innovations where existing products are adapted to the local context and are, thus, new to a country or a region; promotes new organizational and/or technological capabilities; and, promotes innovations in mature markets. Public procurement of innovation requires more radical approaches than GPP and SPP [29], resulting in a complex and interactive process where there is a key activity of learning by interaction [9]. The most immediate practical implication is that evolving from GPP to SPP and from SPP to PPI, requires public organizations in general and public authorities in particular, to have an improved approach, involving an increasing complexity of institutionalized practices and skills, as has been already demonstrated elsewhere [13]. On the other hand, GPP, SPP and PPI practices need to be fully integrated into organizational strategies, in order to allow the significant shift from \"purchase-cost\" to \"life-cycle cost\" approaches, as well as to carry out the corresponding cultural, managerial and operational changes required to effectively contribute to sustainability. These particular needs were stressed by [PERSON] et al. [5] and [PERSON] and [PERSON] [7] for GPP, by [PERSON] et al. [17] for SPP and by [PERSON] and [PERSON] [16] for PPI. Having into consideration the required implementation conditions to overcome the slow uptake of GPP, SPP and PPI, [PERSON] et al. [5] argue that the adoption of guidelines and tools can assist organizations in becoming more GPP oriented.