An expandable reamer for a borehole has a plurality of support elements (202) distributed around a central axis of the tool with cutters (234, 236) and stabilising pads (250) on the support elements, arranged such that at least one of the support elements comprises at least one region comprising cutters and at least one support element comprises a stabilising pad having a front surface positioned to face the wall of the borehole. The reamer includes drive mechanism for expanding the support elements (202) radially outwardly from the central axis. The support elements differ in the presence or axial positions of stabilising pads (250) thereon such that at least one support element differs from at least one other support element in the presence or axial position of at least one stabilising pad. This arrangement allows the stabilising pads to be positioned at or close to full gauge, thereby reducing vibration.