The first edition of this book was published in 1975. Demand and acceptance prompted a second edition in 1978. The third edition published in 1984 was a fairly enlarged edition with updated information on various aspects of principles of plant pathology with a futuristic approach. Between 1984 and 2000 the third edition had 14 prints. Readers suggested that the chapter on plant disease management was too exhaustive and could form subject matter for a new title, Plant Disease Management. This was done. The rapid advances in concepts of different aspects of plant pathology since 1984 have compelled the present revision and expansion of the book. To avoid repetition, the chapter on plant disease management is condensed. At the same time new information on epidemiology, host-parasite relationship and genetic and molecular aspects of host-parasite interaction have been incorporated.