ABSTRACT
The role of networks in innovation processes has become a key research area in the field of innovation studies over the last decade and a half (Freeman 1991; Powell et al. 1996; Hagedoorn 2002; Owen-Smith and Powell 2004; Ahuja et al. 2009). Not surprisingly, the rapid increase in the number of studies on innovation networks in an inter-disciplinary field, such as innovation studies, has led to a great variety of theories and concepts (Ozman 2009). Only recently, economic geographers have jumped on the study of the spatial dimensions of social networks in innovation processes (Ter Wal and Boschma 2009), following the literature on national and regional innovation systems developed in the 1990s (Freeman 1987; Nelson 1993; Cooke et al. 1998; Cooke 2001). Despite this attention, theoretical accounts of spatial networks are still underdeveloped (Grabher 2006; Sunley 2008; Hess 2008). This is also true for an evolutionary approach to knowledge and innovation networks, although attempts have been undertaken more recently (Giuliani and Bell 2005; Powell et al. 2005; Cantner and Graf 2006; Sorenson et al. 2006; Boschma and Ter Wal 2007; Giuliani 2007; Glückler 2007; Morrison 2008; Suire and Vicente 2009; Balland et al . 2010; Breschi et al. 2010; Cassi and Plunket 2010; Glückler 2010; Graf 2010; Balland 2011; Broekel and Boschma 2011; Ter Wal 2011; De Vaan 2012).
