Abstract: An apparatus for determining preferably Biologically Relevant Measurement Variables at membranes has at least one first plate-like or film-like substrate, which is made from at least one first material and has continuous openings or pores or can be perforated, and also has a second plate-like or film-like substrate which is made from at least one second material, to which measuring elements are assigned or on which measuring elements are situated. Cells or similar structures may be at least temporarily applied or permanently cultivated on the first substrate. According to the invention, the first substrate and the second substrate are assigned to one another at least during the measuring operation for determining the corresponding measurement variables, specifically preferably on flat sides which are opposite one another. In particular, this assignment is a connection of the two substrates, preferably a joining-together.