Prof. Leon Mestel has been an inspiration to many to study the role of magnetism in the Cosmos. To mark the occasion of his retin'ment from the University of Sussex after 43 years in astrophysics, several of his friends and former students decided to hold an advanced research workshop in his honour. NATO agreed to finance this venture which was held at the Institute of Astronomy at Cambridge. The scientific organizing committee was J. Landstreet, D. Lynden-Bell, F. Pacini, M.A. Rud0rman and N.O. Weiss and most leading experts on Cosmical magnetism agreed to come. We are particularly grateful to Lyman Spitzer who, ably helped by his wife Doreen, !!;ave the after dinner addre~s on how the goddess Astrophysica had foreseen Leon's achievements in classical Greek times. Not without regret we decided to maintain the homog0neity of the material and therefore could not cover Leon Mestel's major achievements in non-magnetic astronomy. His work on the cooling of white dwarfs, his understanding that degenerate hydrogen was a nuclear explosive since its pressure was almost independent of temperature and hence, his picture of supernovae, which is now more commonly applied to novae, his seminal understanding of the 'law' of galactic rotation and his work on the non-linear development of t hp anisotropies generated in gravitational collapse.

Prof. Leon Mestel has been an inspiration to many to study the role of magnetism in the Cosmos. To mark the occasion of his retin'ment from the University of Sussex after 43 years in astrophysics, several of his friends and former students decided to hold an advanced research workshop in his honour. NATO agreed to finance this venture which was held at the Institute of Astronomy at Cambridge. The scientific organizing committee was J. Landstreet, D. Lynden-Bell, F. Pacini, M.A. Rud0rman and N.O. Weiss and most leading experts on Cosmical magnetism agreed to come. We are particularly grateful to Lyman Spitzer who, ably helped by his wife Doreen, !!;ave the after dinner addre~s on how the goddess Astrophysica had foreseen Leon's achievements in classical Greek times. Not without regret we decided to maintain the homog0neity of the material and therefore could not cover Leon Mestel's major achievements in non-magnetic astronomy. His work on the cooling of white dwarfs, his understanding that degenerate hydrogen was a nuclear explosive since its pressure was almost independent of temperature and hence, his picture of supernovae, which is now more commonly applied to novae, his seminal understanding of the 'law' of galactic rotation and his work on the non-linear development of t hp anisotropies generated in gravitational collapse.

The visible universe is a small perturbation on the material universe. Zwicky and Sinclair Smith in the 1930s gave evidence of invisible mass in the Coma and Virgo Clusters of Galaxies. Better optical data has only served to confound their critics and the X-ray data confirms that the gravitational potentials are many times larger than those predicted on the basis of the observed stars. Dynamical analyses of individual galaxies have found that significant extra mass is needed to explain their rotational velocities. On much larger scales, tens of megaparsecs, there is suggestive evidence that there is even more mass per unit luminosity. What is this non-luminous stuff of which the universe is made'? How much of it is there? Need there be only one kind of stuff? There are three basic possi bili ties:- all of it is ordinary (baryonic) matter, all of it is some other kind of (non-baryonic) matter, or some of it is baryonic and some is non-baryonic.