Plants represent a rich source of nutrients for many organisms including fungi, bacteria, protists, insects, and vertebrates and constantly being challenged by various types of pathogens, but disease is rare. Why? In general, there are three reasons for pathogen failure. Either a) The plant is unable to support the niche requirements of a potential pathogen and is thus a non-host b) The plant possesses preformed structural barriers like wax, cuticle/some type of structure on natural opening or toxic compounds that confine successful infections to specialized pathogen species c) Upon recognition of the attacking pathogen, defense mechanisms are elaborated and the invasion remains localized because of hypersensitive response All three types of interaction are said to be incompatible, and depend upon preexisting or induced defense mechanism play by plant. Successful pathogen invasion and disease (compatibility) ensue if the preformed plant defenses are inappropriate, the plant does not detect the pathogen, or the activated defense responses are ineffective. So this chapter intended to describe the essential prerequisites for pathogen recognition and the induction of localized defense responses. Because it is important to better recognize the mechanisms through which plants use to resist the infection of pathogen or to establish an effective defense response, as a better and thorough apprehension of the defense mechanisms of such interactions will ultimately minimize the impact of most of the devastating plant diseases.