A flywheel assembly (10; 40; 100) comprising: a flywheel mass having substantially parallel and spaced first and second surfaces (13,15), and a plurality of openings extending through the mass from said first surface to said second surface in a direction substantially parallel to a rotational axis about which the mass can rotate in use; a first flywheel mass support (14; 20; 102) comprising a first surface configured to abut against the first surface of said flywheel mass, said first flywheel mass support further comprising a shaft (19; 106) that extends along the rotational axis about which the first flywheel mass support (14; 20) can rotate in use, and a plurality of openings that are each offset from said rotational axis and located in close proximity to a radially outward periphery of the first support (14; 20); a second flywheel mass support (20; 104) having a first surface configured to abut against the second surface of the flywheel mass, said second flywheel mass support comprising a plurality of openings that are each offset from said rotational axis and located in close proximity to a radially outward periphery of the second support; and means (23; 42, 44) for coupling said flywheel mass to the first and second supports (14; 20) so that the mass can rotate with the supports in use, said coupling means being configured to extend through the aligned openings in the supports and said flywheel mass; wherein said flywheel mass comprises a plurality of generally planar flywheel mass elements sandwiched together to form a stack of elements, each said element including a plurality of openings that align with the openings in said first and second mass supports (14; 20) and with openings in neighbouring elements in said stack, the openings in said mass being offset from said rotational axis and located in close proximity to a radially outward periphery of said mass, said flywheel elements being coupled together and aligned with one another to form said flywheel mass and being coupled to said first and second supports (14; 20) solely by means of the coupling means (23; 42, 44) that extends through the aligned openings in said elements and said first and second supports (14; 20); characterised in that the openings in said mass are non-circular, and each comprise a primary region and a pair of auxiliary regions, one auxiliary region of said pair located to one side of said primary region and the other auxiliary region of said pair located to the other side of said primary region, each auxiliary region opening into said primary region. a flywheel mass having substantially parallel and spaced first and second surfaces (13,15), and a plurality of openings extending through the mass from said first surface to said second surface in a direction substantially parallel to a rotational axis about which the mass can rotate in use; a first flywheel mass support (14; 20; 102) comprising a first surface configured to abut against the first surface of said flywheel mass, said first flywheel mass support further comprising a shaft (19; 106) that extends along the rotational axis about which the first flywheel mass support (14; 20) can rotate in use, and a plurality of openings that are each offset from said rotational axis and located in close proximity to a radially outward periphery of the first support (14; 20); a second flywheel mass support (20; 104) having a first surface configured to abut against the second surface of the flywheel mass, said second flywheel mass support comprising a plurality of openings that are each offset from said rotational axis and located in close proximity to a radially outward periphery of the second support; and means (23; 42, 44) for coupling said flywheel mass to the first and second supports (14; 20) so that the mass can rotate with the supports in use, said coupling means being configured to extend through the aligned openings in the supports and said flywheel mass; wherein said flywheel mass comprises a plurality of generally planar flywheel mass elements sandwiched together to form a stack of elements, each said element including a plurality of openings that align with the openings in said first and second mass supports (14; 20) and with openings in neighbouring elements in said stack, the openings in said mass being offset from said rotational axis and located in close proximity to a radially outward periphery of said mass, said flywheel elements being coupled together and aligned with one another to form said flywheel mass and being coupled to said first and second supports (14; 20) solely by means of the coupling means (23; 42, 44) that extends through the aligned openings in said elements and said first and second supports (14; 20); characterised in that the openings in said mass are non-circular, and each comprise a primary region and a pair of auxiliary regions, one auxiliary region of said pair located to one side of said primary region and the other auxiliary region of said pair located to the other side of said primary region, each auxiliary region opening into said primary region.