Abstract: A rotor such as a blisk B is balanced by determining the extent of any static or couple unbalance when the rotor is rotated about a test axis X while supported at a location surface 10 at which the rotor is supported in normal use. The location surface 10 is then modified, for example by removing material in a final machining operation, to eliminate the unbalance. For example the swash of the surface 10 may be altered and/or modification may be made to other location features (11) in order to cause the rotor to rotate, in normal operation, about an axis which is eccentric to the test axis X.