A flywheel for kinetic energy storage and its construction using composite materials. The present invention provides a flywheel assembly having a longitudinal axis and comprising an annular rotor and a rotor support for coupling the rotor to an axial shaft, wherein the rotor comprises fibers in a matrix material, and a ring comprising fibers in a matrix material is mounted on the outer circumference of the rotor support and the rotor is mounted on the outer circumference of the ring, the rotor, rotor support and ring each having longitudinal axes which are coincident with the longitudinal axis of the assembly. The presence of the intermediate ring formed of a composite material assists in the fabrication of the assembly and increases its durability by providing a suitable interface between the rotor and a rotor support.