The paper analyses challenging the validity of an employment contract by the Social Insurance Institution, in the course of a control of correctness of the determination of the social insurance obligation that results from the employee status. In the first part, the author analyses the problem of a fictitious contract and acting against the law or with the intention to circumvent the law, or contrary to the principles of community life. He indicates documents and information that are most often requested by ZUS, and describes the situations which are most thoroughly examined. In the next part, following issues are examined: the existence of an employment relation, salary, employing relatives, concluding the employment contract in order to register for insurance purposes and the situation of pregnant employees or employees shortly before retirement. The paper is concluded with the most frequent arguments for, or against the existence of employment and the most advantageous practices to litigate with the ZUS are described.