This paper examines how professional associations engage themselves in efforts to develop, regulate and secure knowledge in their respective domains, with special emphasis on standardisation. The general emphasis on science in society brings renewed attention to the knowledge base of professionals, and positions professional bodies as key regulatory agencies. At the same time, knowledge takes distinctive forms in different areas of expertise, and the "knowledge work" of professional associations is embedded in complex settings of actors and interests that need to be negotiated. Based on documents and interviews with core representatives, we examined approaches to standardisation in three associations that represent the main bodies of nurses, teachers and auditors in Norway. The analysis shows that all associations engage themselves in efforts to develop standards for knowledge and professional practice, but that they do so in different ways and with alternative sources of legitimisation. Standardisation is initiated for variegated purposes, and involves the ongoing negotiation of tensions between different concerns. We discuss the approaches taken in relation to conditions for professionalism, and argue that the knowledge work of professional associations is becoming increasingly important in a society where knowledge, as well as the market for professional services, is becoming internationalised.