Firms increasingly rely upon external actors for their innovation process. Interaction with these actors may occur formally (i.e. through a collaboration agreement) or informally (i.e. external actors acts as sources of information). We analyze the reasons why firms consider it to be important to develop formal and informal external linkages in the innovation process by looking at the role played by firms' innovative strategies and by taking into account the possibility that a complementarity or substitutive relationship might exist between formal and informal linkages. Data are from the Third Community Innovation Survey (CIS 3), where we have access to firm level micro-data from Sweden, Norway, the Netherlands and the UK.