Technical Abstract: Instead of being passively attacked by insect pests, plants possess a myriad of defense mechanisms to protect themselves. These mechanisms function broadly either by directly reducing herbivore fitness (direct plant defense), or by indirectly attracting natural enemies of the herbivores (indirect plant defense). Many biotic and abiotic factors including nitrogen affect the expression of plant defenses. Nitrogen fertilization is by and large the most common agronomic practice in crop production. Anthropogenic use of nitrogen meets the food supply for the rapid increase in human population, while it alters plant defense capability and the global nitrogen cycle, which is increasingly threatening plant and animal biodiversity in managed and natural ecosystems. In this review, we first review types of plant defense and ecological and environmental consequences associated with anthropogenic addition of nitrogen into ecosystems. We then review the influences of nitrogen fertilization on plant-herbivore-natural enemy interactions and plant direct and indirect defenses. We finally discuss the possibility of developing new sustainable pest management tactics through nitrogen management: minimizing the negative, and maximizing the positive components of tritrophic interactions through optimized nitrogen applications in agricultural production.