This paper examines the neural organization of the defensive behavior system responsible for fear, using the behavior systems approach. The defensive behavior system is divided into three modes: pre-encounter, post-encounter, and circa-strike. Low levels of fear promote pre-encounter defenses, such as meal-pattern reorganization. Moderate levels of fear activate post-encounter defenses, with freezing being the dominant response in rats. High levels of fear trigger active defenses, such as fighting or fleeing, which are controlled by midbrain structures like the dorsolateral periaqueductal gray (dlPAG) and the superior colliculus. Inhibitory interactions between these structures allow for rapid switching between defensive modes. The behavior systems approach views animals as having genetically determined behaviors that solve functional problems. When threatened, animals use these behaviors to respond to environmental stimuli. The species-specific defense reaction (SSDR) theory suggests that animals use innate behaviors to respond to threats. However, this theory has been challenged by evidence showing that fear responses are not solely determined by environmental stimuli but also by the level of fear. The paper discusses the neural circuits involved in post-encounter defense, including the ventrolateral periaqueductal gray (vPAG) and the amygdala. The amygdala plays a critical role in processing fear-related information and activating freezing responses. The vPAG is involved in autonomic and analgesic responses that support freezing. The dlPAG is involved in active defenses, such as fighting or fleeing, and is connected to the amygdala and other brain regions. The paper also discusses the neural circuits involved in circa-strike defense, which includes active behaviors like jumping or vocalizing. The dlPAG is involved in these behaviors, and its activation is influenced by sensory input and environmental cues. The superior colliculus is also involved in these responses. The paper concludes that the defensive behavior system is organized in a way that allows for rapid switching between different modes of defense based on the level of fear and the nature of the threat. The neural circuits involved in these responses are complex and involve multiple brain regions, including the amygdala, vPAG, and dlPAG. Understanding these circuits is essential for understanding how animals respond to threats and how fear is processed in the brain.This paper examines the neural organization of the defensive behavior system responsible for fear, using the behavior systems approach. The defensive behavior system is divided into three modes: pre-encounter, post-encounter, and circa-strike. Low levels of fear promote pre-encounter defenses, such as meal-pattern reorganization. Moderate levels of fear activate post-encounter defenses, with freezing being the dominant response in rats. High levels of fear trigger active defenses, such as fighting or fleeing, which are controlled by midbrain structures like the dorsolateral periaqueductal gray (dlPAG) and the superior colliculus. Inhibitory interactions between these structures allow for rapid switching between defensive modes. The behavior systems approach views animals as having genetically determined behaviors that solve functional problems. When threatened, animals use these behaviors to respond to environmental stimuli. The species-specific defense reaction (SSDR) theory suggests that animals use innate behaviors to respond to threats. However, this theory has been challenged by evidence showing that fear responses are not solely determined by environmental stimuli but also by the level of fear. The paper discusses the neural circuits involved in post-encounter defense, including the ventrolateral periaqueductal gray (vPAG) and the amygdala. The amygdala plays a critical role in processing fear-related information and activating freezing responses. The vPAG is involved in autonomic and analgesic responses that support freezing. The dlPAG is involved in active defenses, such as fighting or fleeing, and is connected to the amygdala and other brain regions. The paper also discusses the neural circuits involved in circa-strike defense, which includes active behaviors like jumping or vocalizing. The dlPAG is involved in these behaviors, and its activation is influenced by sensory input and environmental cues. The superior colliculus is also involved in these responses. The paper concludes that the defensive behavior system is organized in a way that allows for rapid switching between different modes of defense based on the level of fear and the nature of the threat. The neural circuits involved in these responses are complex and involve multiple brain regions, including the amygdala, vPAG, and dlPAG. Understanding these circuits is essential for understanding how animals respond to threats and how fear is processed in the brain.