This article is based on a plenary address which Robert W. Cahn, Cambridge
University, delivered at the International Union of Materials Research Soci
eties International Conference on Advanced Materials (IUMRS-ICAM'99) hosted
by the Chinese Materials Research Society (C-MRS) in Beijing, oil June 14,
1999. A version of this article is also being published in Materials Today
, the quarterly magazine published by Elsevier for the European-MRS. A repo
rt of the IUMRS-ICAM'99 meeting will he published in a future issue of MRS
Bulletin.