The long-term target of improving crops resistance to biotic stresses is a familiar goal for breeders. Plants ought to constantly protect themselves versus aggressions from a wide spectrum of organisms that include viruses, bacteria, oomycetes, fungi, insects and other herbivores, and weeds. In this chapter attention will be given to depict a picture on the genetic and molecular mechanisms that plants have promoted to recognize and react to invasion by numerous parasites (pathogens and pests). These topics include non-host resistance, constitutive barriers, and race-specific resistance. The chapter also examines current progresses in clarifying the structure and molecular devices developed by plants to neutralize pathogen and pest aggressions. Moreover, it takes a look with aspects experienced in breeding for resistance to relevant biotic stress factors. Major considerations in breeding for resistance to pathogens, insect pests, and weeds, traditional sources of resistance or other possible strategies, such as mutation breeding, genetic manipulations, and molecular strategies to develop crops more resistant to parasites are also explored.