Over the past few years, the number of reports of roots cultured in reactors has increased dramatically. This has provided much-needed information on the physical and metabolic requirements of cultured roots, and has established techniques for the manipulation of root tissue. Most of this work, however, is conducted on a relatively small scale (less than 5 l), and many are demonstrations as opposed to systematic studies. The real challenge of scale-up will be encountered when moving to the 100-1000 l scale, while maintaining dry tissue densities in excess of 10-20 (g dry weight) l-1. Achievement of this goal will require small-scale studies, focused on obtaining design and operational information, coupled with measurements of fundamental scale-up parameters in large-scale systems.