Most modern diffraction gratings are metal coated plastic film replicas, on rigid substrates, that were taken from a master. Master gratings are made either by mechanical ruling, where uniformly spaced grooves are burnished in soft metal films by a diamond tool, or by exposing a photoresist coated blank to an interference fringe field generated with the aid of high powered lasers. Basic techniques are described for plane and concave gratings. Performance tests are concerned primarily with diffraction efficiency and with perfection of the diffracted wavefront. Imaging defects show up in reduced resolution. Groove spacing effects show up as ghosts, stray light and satellites. These will be reviewed.