Since in the Netherlands only about a quarter of the employees is a member of a trade union, collective labour agreements are, in the majority of cases, legally binding not by law (i.e.: trade union members are automatically bound), but only by way of incorporation clauses in the labour contracts of employees. In this way, about 85% of employees is bound by the provisions of a collective labour agreement. The interpretation and application of such clauses have, in legal practise, at least partially a non-contractual character in order to accommodate other interests than the employee's freedom of contract. The author analyses legal practise regarding this matter and concludes that the non-contractual approach of incorporation clauses is under pressure, which threatens the system of collective labour law.