The levels of disease caused by a particular pathogen species on its host plant can vary considerably. This has both fascinated plant pathologists and been exploited by plant pathologists and plant breeders for over a century. Different isolates of the pathogen can be more or less aggressive, and different levels of host susceptibility can be observed. This chapter delves into the molecular basis underlying the genetic variation observed in the host and pathogen that affect disease intensity. The various levels of interaction at the cellular and molecular level now known as plant immunity, which in essence represents the arms race or coevolution of host and pathogen, are also presented.