The information about molecular structure that may be obtained from a study of the intensity-dependence of the optical rotation angle is discussed. It is shown that there are two contributions: one depends upon the relative disposition and anisotropies of the polarisability and gyration tensors, whilst the other reflects the contribution of excited states of the molecule. The non-linear contribution is compared with the effect that an externally applied electric field would have on the rotation angle, and the relative advantages and disadvantages are discussed.