This paper applies the behavior systems approach to fear and defensive behavior, examining the neural circuitry controlling these behaviors. The defensive behavior system is divided into three modes: pre-encounter, post-encounter, and circa-strike, each activated by different levels of fear. Low levels of fear promote pre-encounter defenses, such as reorganizing meal patterns. Moderate levels of fear activate post-encounter defenses, with freezing being the dominant response in rats. The amygdala plays a critical role in organizing this mode, with projections to the ventral periaqueductal gray (vPAG) activating freezing. High levels of fear, such as physical contact, elicit the vigorous active defenses of the circa-strike mode, involving midbrain structures like the dorsolateral periaqueductal gray (dlPAG) and the superior colliculus. Inhibitory interactions between structures mediating different modes allow for rapid switching between defensive responses. The paper outlines a response-selection rule based on predatory imminence, which influences the choice of defensive behavior. It also describes the neural organization of SSDRs and how this allows for effective switching between them. The analysis highlights the importance of understanding behavior at multiple levels to gain a comprehensive understanding of behavior and biology.This paper applies the behavior systems approach to fear and defensive behavior, examining the neural circuitry controlling these behaviors. The defensive behavior system is divided into three modes: pre-encounter, post-encounter, and circa-strike, each activated by different levels of fear. Low levels of fear promote pre-encounter defenses, such as reorganizing meal patterns. Moderate levels of fear activate post-encounter defenses, with freezing being the dominant response in rats. The amygdala plays a critical role in organizing this mode, with projections to the ventral periaqueductal gray (vPAG) activating freezing. High levels of fear, such as physical contact, elicit the vigorous active defenses of the circa-strike mode, involving midbrain structures like the dorsolateral periaqueductal gray (dlPAG) and the superior colliculus. Inhibitory interactions between structures mediating different modes allow for rapid switching between defensive responses. The paper outlines a response-selection rule based on predatory imminence, which influences the choice of defensive behavior. It also describes the neural organization of SSDRs and how this allows for effective switching between them. The analysis highlights the importance of understanding behavior at multiple levels to gain a comprehensive understanding of behavior and biology.