Abstract: An insertion part is provided in the form of a back-flow preventer and which can be inserted in a gas or liquid line. An insertion part is also provided that can be inserted into a gas or liquid line in the form of a flow regulator. The insertion parts are characterizedin that they comprise an annular lip shaped part which, togther with its annular body, is held inside the housing and which has at least one controlling lip (9), with the controlling lip being displaced by the fluid, and/or at least one sealing lip (14). The insertion part can be used in a versatile manner and is characterized by the simple production thereof as well as by having a uniform and reliable functioning.