Although the increase of knowledge as an 'asset' in products and services offers new challenges and opportunities, it also throws up new barriers. One such barrier, the lack of access to knowledge, is analysed as consisting of several processes with different key factors that obstruct or stimulate those processes. Furthermore, different levels of analysis are introduced and the concept of 'knowledge circulation' is positioned. Barriers to access affect in particular small and medium-sized enterprises (SMEs), which consequently do not participate to their full potential in the knowledge infrastructure. A new initiative in the Netherlands, the Innovation Alliance, focuses on renewing and strengthening the position of SMEs in the knowledge infrastructure giving special attention to the universities of professional education and their 'centres of excellence'. This is illustrated by a project in which SMEs work together in a hydraulic cluster.