The author describes the functions and limits of the employer's power of organising the workers' jobs within the enterprise. This power is mediated by the labour contract, which is a primary instrument of control and limitation of the employer's powers, but it goes beyond it as an instrument of organisation of the enterprise. The employer's directive power is examined as regards both the definition of the employment relationship, i.e. the so called operation of qualification, and its primary function of managing the employment relationship, i.e. as an instrument of functional flexibility, which finds however its limits in the necessity of protecting the worker's personal rights. One of the limits to the employer's directive power can be found in the principle of rationality and the author ends his paper asking himself whether or not Labour Law is ready to embody this principle.