A method for the control of the transmission of bending vibrations in beams is described. A composite beam (a structural insert) is inserted into a part of the structure to affect the transmission there. The insert comprises two elastic face plates and a central core of tunable fluid such as an electrorheological (ER) or magnetorheological (MR) fluid. By adjusting the applied electric or magnetic field the rheological properties of the fluid can be controlled and hence so too can the vibration transmission properties of the insert. Theoretical results concerning the transmission of vibrations in beam-like structures are reviewed. Numerical results are presented for the transmission through ER and MR fluid filled inserts, with experimental results for an ER case. The transmission is seen to show a pass/stopband structure with the stopbands being tunable.