Poultry is the most widely farmed land animal, comprising over 80% of the main livestock populations. The high prevalence of injurious pecking (IP) in poultry is a major concern from animal welfare, societal, and economic points of view. IP is defined as bird-to-bird pecking that results in or has a high likelihood of causing integument injury and psychological harm to the victim. In this chapter, we describe three forms of IP – tissue pecking (TP), aggressive pecking (AP), and feather pecking (FP). Furthermore, it explores the two major views explaining the origin of severe FP, the most prevalent form of IP. The first, the ethological view, emphasizes the role of the environment in creating motivational frustration. In contrast, the dysfunctional view identifies underlying neurobiological dysfunctions induced by intense or sustained stress as the cause of severe FP. Finally, we discuss risk factors and management strategies that are used to reduce IP.