This book presents a collection of papers that give a comprehensive description of the major areas of research on the interactions between plants and their pathogens. State-of-the-art knowledge about research on microbial avirulence genes and plant resistance genes, about pathogenicity factors and signals for establishment of infection, about elicitors and intermediates in the signalling pathway, and about the genes involved in plant responses to pathogens and environmental stress is reviewed. The different topics covered show that modern tools of plant cellular and molecular biology are now available to investigate very significant models of plant-pathogen interactions and to identify the molecular determinants of the two partners that trigger defence responses, the signals involved and the mechanism of their transduction, the spatial and temporal regulation of defence gene expression in relation to the spread or localization of the pathogen. The first trials to exploit these results for enhancing plant resistance and crop field by biotechnology are presented. The book is a main reference source for research scientists and advanced students.