Cool data: quantity AND quality.

The use of cryo-techniques in macromolecular crystallography has increased enormously over the last eight years and has become a vital part of modern X-ray data-collection methods. This paper presents some reasons for the rise in popularity of cryo-techniques and a brief outline of the basic methods, followed by a detailed discussion of factors to be considered when trying to optimize both the quantity and quality of the data collected. As more experimenters at synchrotrons observe significant radiation damage to crystals held near 100 K, the available options for further prolonging crystal lifetime and extending the techniques become worth investigating. Some possibilities and parameters to be considered are presented, although these must remain speculative until more experimental data are available.