As befits a volume in the Advanced Series in Agricultural Sciences, this book was written with problems of practical agriculture in mind. One of the ways of controlling plant disease is by using resistant cultivars; and from the wide literature of genetics and biochemistry in plant pathology I have emphasized what seems to bear most closely on breeding for disease resistance. This has a double advantage, for it happens all to the good that this emphasis is also an emphasis on primary causes of disease, as distinct from subsequent processes of symptom expression and other secondary effects. The chapters are entirely modern in outlook. The great revolution in biology this century had its high moments in the elucidation of the DNA double helix in 1953 and the deciphering of the genetic code in 1961. This book, so far as I know, is the first in plant pathology to be conceived within the framework of this new biology. Half the book could not have been written 20 years ago, even if there had then been available all the literature that has since accumulated on the genetics and chemistry of plant disease. The new biology is the cement this book uses to bind the literature together. Another feature of this book is an emphasis on thermodynamics.