1. In a tracking system configured to track a face/head, a method of tracking facial features of the face/head, the method including the step of: (a) determining if glasses rims, frames or arms are present on the face/head; and (b) where said rims, frames or arms are present on the face/head, determining appearance characteristics of the rims, frames or arms thereby to derive 2D image position or 3D pose measurements of the glasses or the face/head, wherein deriving the 2D image position or 3D pose measurements includes performing analysis thereby to progressively optimise the fit of a three dimensional generative glasses model to a progressive set of sequential input images containing the face/head, wherein the fit is defined in terms of glasses shape parameters and glasses pose parameters.