Plant disease is one of the greatest causes of economic losses of crops throughout the world. Every year, billions of dollars are spent on chemical control measures, but there is an increasing awareness that alternatives to these chemical control measures may be becoming more viable. This work explores the international impact of plant disease on man and how this is measured and diagnosed, discusses epidemiology and the genetics of host-parasite relations and looks at the range of defensive tactics currently available and those under development, in particular those which are more environmentally acceptable than many in widespread use now.