The task of market surveillance and in-service-inspection is to ascertain if at any transaction liable to mandatory verification verified measuring instruments are used, i.e. instruments with valid verification mark or conformity assessment mark. Whereas market surveillance supervises the initial placing on the market of measuring instruments, in-service-inspection aims at measuring instruments in use. Furthermore, these supervising measures have to ensure that not permissible measuring instruments are put out of operation, that the legal interval of subsequent verification is observed and that the regulations which ensure correct use of verified instruments in all transactions liable to mandatory verification are adhered to. Therefore, market surveillance and in-service-inspection are an essential complement to verification, only both together give the protection of public that is the aim of the Metrology Act.