An apparatus for determining and/or monitoring at least one process variable of a medium, comprising: an oscillatable unit, which has a membrane and at least one oscillatable element, wherein the oscillatable element is secured to the membrane at least in a first securement region and in a second securement region. At least one driving/receiving unit, which excites the oscillatable unit to execute mechanical oscillations and which produces a received signal dependent on the oscillations of the oscillatable unit; and a control/evaluation unit, which evaluates the received signal with reference to the process variable. The apparatus is distinguished by features including that the driving/receiving unit is embodied in such a manner and arranged on a rear face of the membrane facing away from the oscillatable element that the oscillatable element executes torsional oscillations.