A rotor structure for an interior permanent magnet electric motor comprising: A laminated rotor core which comprises stack of thin plates, the stack having a first end and a second end, each plate in the stack including at least one window which is aligned with a corresponding window of an adjacent plate so that the window together form an elongate pocket that extends along at least a part of the length of the core starting at a first end of the core, and at least one permanent magnet located in the pocket. At least one plate at the first end of the stack includes an integral retention feature which projects away from a main body of the plate to project above the window in the plate and which engages the first end of the magnet thereby to retain the magnet in the pocket.