An implantable device for sensing intravascular pressure, the device comprising a bulk acoustic wave (BAW) resonator arranged to be deflected by changes in intravascular fluid pressure to provide a pressure dependent sensing signal and an acoustic reflector arranged to separate the BAW resonator from the intravascular fluid and to mediate pressure from the intravascular fluid to the BAW resonator wherein the acoustic reflector comprises a layer structure arranged to provide a series of transitions in acoustic impedance between the intravascular fluid and the BAW resonator.