The interactions of fungi with mankind are both beneficial and harmful and are deeply rooted in the history of human society and agriculture. Over the centuries humans have sought to manipulate the growth of fungi to their advantage; the methods used, though largely empirical, have often been highly successful. Since the initial development of recombinant DNA technology in bacteria in the early 1970s, biology has been undergoing a molecular revolution which is spreading to all organisms, including fungi. The approach and techniques of molecular biology enable us to ask and answer fundamental questions about many aspects of fungal biology, and open the way to the directed manipulation of fungal metabolism. This book highlights the rapid development of gene transfer and cloning techniques in fungi and the ways in which these are being exploited in species of economic importance either in biotechnology or as plant pathogens.