Most optical systems are designed on the assumption that the optical component's performance is set soley by the functional specification i.e. that the shape, form, finish and refractive index of the component material. Lip service, at best, is paid to the fact that all materials have an absorption coefficient that dominates these parameters. Very little thought is given to the effect of the holder, that the absorption may not be spatially uniform or that the resulting temperature rise may be non-uniform, to birefringence, to changes in the refractive indices or to lowered damage thresholds, These effects in turn affect the beam propagation characteristics of the beam. All these effects have been observed in a variety of different laser systems. A series of examples are recorded to illustrate the theme and to illustrate the further contention that the performance of optical components needs to be tested in the systems in which they will be finally used. It will then follow that, in order to obtain agreement between optical component manufacturers and system manufacturers and users, standardised measurements have to be made.