On the one hand, this chapter explains why, although in many cases migrant workers are subject to the social security legislation and to labour law of the same Member State, such synchronism is no guarantee. As a matter of fact, the conflict of law rules in both fields vary in their detail. In addition, they are based upon different principles. As a result, a migrant worker may be subject to the social security legislation of one Member State and to labour law of another Member State. Our analysis shows that this is especially true for posted workers who are posted for a period longer than two years and for workers who normally pursue an activity in two or more Member States. This could lead to problems for the worker concerned and his employer, for instance in case of sickness of the worker. On the other hand, this chapter exposes why, only by way of exception, elements of labour law are also covered by the social security Regulation 883/2004. As is explained, this does not mean, however, that migrant workers are not protected at all in these matters. It is indeed for the national administration and judiciary to interpret and apply domestic law in conformity with Union law. Their task is certainly not an easy one.