Barbara McClintock was born in 1902, within a few years of the rediscovery of Mendel's laws. Her life, discoveries, and insights span the history of genetics in this century. McClintock's unique ability to discern relationships between the behavior of chromosomes and the properties of the whole organism earned her early recognition. At Cold Spring Harbor, she began the studies of the consequence of dicentric chromosome formation and breakage that led her to the discovery of genetic elements capable of moving within the genome and controlling expression of other genes. This book contains a kaleidoscope of contributions, many by those who discovered transposition in other organisms. Their essays give a remarkable account of the scientific legacy of one of the century's greatest geneticists.