The rotation of the Galaxy can best be studied with the aid of tracers for which distance and velocity can be obtained simultaneously, accurately and unambiguously out to large distances. Clearly, the best objects are the very young clusters. The local gradient of the angular velocity is contained in Oort's constant
and, due e.g. to perturbations from spiral features, may not reflect the smoothly varying, underlying global component of rotation. Thus, it is necessary to reach out as far as possible in order to allow a smoothing of the rotation curve.