This article proposes a definition of inter-organisational trust. In particular, it shows how boundary spanners, by virtue of their belonging to communities of practice, can play a key part in the emergence of this form of trust. The argument is based on a case study of sub-contracting in the Lyons mechanical construction industry. The analysis shows that inter-organisational trust is grounded in inter-personal trust, since its emergence depends critically on relations of trust that buyers establish with sales agents in the sub-contractor firms. Yet these intermediaries' behaviour is not independent from the professional context in which they operate. They belong to "communities of practice" or groups of individuals united by their engagement in the same type of activity. These communities of practice have created an institutional context in the area of Lyons, favourable to the emergence of a new form of sub-contracting based on a high level of trust.