Over the last fifteen years, we have witnessed the growth of various ''atypical'' forms of employment and a growing interest in part-time work, piecework and subcontracting. This trand has given rise to two very different types of employment on the labour market: regular and permanent jobs versus part-time or temporary jobs. The latter often include contract employees from employment agencies hired on a temporary basis. A firm dealing with such agencies avoids the cost of hiring regular workers and benefits from greater flexibility as far as personnel management is concerned. Our labour laws are conceived and administered on the assumption that there is a bilateral agreement between employer and employee, in a stable job environment. This two-party relationship has an impact on collective and individual working relations. The distinctive criterion of this working relationship is, as determined by doctrine and jurisprudence, legal subordination, which can be defined as the right to manage, control and correct the activities of another person. But does this criterion apply to a three-party relationship? The notion of legal subordination is always useful to determine who the employer is. However, the procedure used in order to decide who exercises authority is based on increasingly flexible criteria. In the light of trands in jurisprudence, Blouin (1995) suggests that legal subordination can be determined be referring to either criteria or factors, and the method used depends upon the type of relation between the parties. In the scope of the proposed analysis, the following tests are used: the control test; the work organization test; the economic activity test; the fourfold test; and variable factors. In this article, we observe that in cases where the definition of the relationship between employer and employee was problematic, in the context of private placement agencies, the legal subordination criterion prevailed, that is, the firms dealing with those agencies were mainly identified as the employer. The authors consider that, in a three-party relationship, the efficiency test is much too strict and overlooks some fundamental aspects of the question. It is suggested that criteria such as recruitment, selection, daily supervision, compensation, equipment ownership, task assignment, work scheduling, training, work evaluation, discipline, dismissal, length of service, integration in the firm and legal subordination should be looked at more closely by the courts in their search for the genuine employer.