Aggression is defined as hostile behavior directed at an individual. All behavioral choices are made in context with the situation; hence, some aggression is considered a normal response while other instances are considered abnormal. It is important to consider the context as the aggressor interprets it. Reported dog bites in the United States each year number in the millions and account for a significant concern for public health. While rarely fatal for the victims, aggression is a common cause of abandonment, surrender of animals to shelters, presentation to a trainer or behavioral specialist, or euthanasia. Aggression and other behavioral problems may be the leading cause of death for dogs and cats today because owners elect euthanasia for these animals.

The limbic system of the brain controls behavior and emotion. The anatomic location is within the cerebral cortex and brain stem and it includes the amygdala, hippocampus, cingulate gyrus, hypothalamus, and thalamus. Pathologic processes such as inflammation, neoplasia, and toxins may alter the function of the limbic system, and seizure activity originating in this region may result in abrupt and dramatic behavioral changes after which the animal returns to its normal behavior. It should be noted that most animals that are presented for aggression disorders have no identifiable lesion or neurologic dysfunction; therefore, the exact pathogenesis remains unknown in many cases.

Behavioral choices are made in context as the animal perceives the situation. Past experiences are recalled to influence the current reactions. Other things such as state of arousal, level of anxiety or fear, and the overall health of the animal also influence this choice.