Abstract: A hydraulic seal (15) for providing a seal between two concentric shafts (10,12) of a gas turbine engine comprises a radially outer element (16) that defines an annular oil reservoir (21) and a radially inner element (22) having an annular fin (24) that projects into the oil reservoir (21). The radially inner seal element (22) is attached to its associated shaft (10) by frangible shear pins (26). In the event of relative axial movement between the shafts (10,12) the shear pins (26) break, thereby ensuring that such shaft relative axial movement is not unduly inhibited.