behaviour. Control of impurities, by techniques such as wall conditioning, magnetic divertors, pellet or atomic beam injection and radiation mantles, offers a wealth of future investigations. Acknowledgements The author would like to acknowledge the help and inspiration he has derived from his students past and present in writing this article. In particular he is indebted toM O'Mullane for his technical help in preparing the manuscript and whose research work is featured in the sections on MARFEs and ion transport. References Abbey, A. F., Barnsley, R., Dunn, J., Lea, S. N. and Peacock, N.J.: 1993, UVand X-ray Spectroscopy of Laboratory and Astrophysical Plasmas. (editors, E Silver and S. Khan) Cambridge University Press, 493. Afrosimov, V. V., Gordeev, Y.S. et al.: 1979, J.E. T.P. Lett. 28, 501. Alper, B.: 1995, p.r.ivate communication, JET.