In the early 2000s, within only a few years, French environmental non-governmental organisations came to use a larger array of skills, as well as more elaborated strategies of advocacy and fundraising. The evolution of five high-profile organisations is examined in order to question what drives their professionalisation. This empirical question justifies addressing a debate on institutionalisation that is often synonymous with this process of professionalisation. Building on the work of Philip Selznick, institutionalisation is considered to be a key concept to track change within organisations and beyond individual cases. In order to better capture the importance of organisational logics and margins for innovation, longitudinal analysis follows these organisations’ distinctive competences over time when dealing with both external and internal pressures.