The long-term goal of crop improvement for biotic stress tolerance in plants is a traditional objective of breeders. Plants must continuously defend themselves against attacks from bacteria, viruses, fungi, invertebrates, and even other plants. This chapter will summarize the benefits and drawbacks of resistance versus chemical protection. We will provide a description of the effective genetic and molecular mechanisms that plants have developed to recognize and respond to infection by various pathogens and pests. These mechanisms include non-host resistance, constitutive barriers, and race-specific resistance. Recent advances in elucidating the structure and molecular mechanisms used by plants to cope with pathogens and pest attacks will also be covered. Additionally, we will address the most relevant problems in breeding for resistance to parasites, including specificity of defense mechanisms, specificity of parasitic ability, inheritance of resistance, gene-for-gene interaction, and durability of resistance. Major considerations in breeding for resistance to parasites, conventional sources of resistance, and possible alternatives (such as mutation breeding, genetic manipulations, tissue cultures, and molecular interventions) will be discussed Panigrahi, J et al. (2013).