Defensive behavior occupies a central position in the depth-psychology and neurobiological literature on trauma, fear, and survival. The corpus reveals a field structured around a core tension: whether defensive responses are best understood as neurobiological survival circuits (LeDoux), as somatically encoded action tendencies that persist pathologically after trauma (Ogden, Levine), or as phylogenetically conserved animal defense patterns that serve as the primary model for understanding dissociation (Nijenhuis). LeDoux situates defensive behavior squarely within nonconscious defensive survival circuits, emphasizing the sequential architecture of freezing, fleeing, and fighting as hardwired response programs modulated by amygdala and BNST circuitry. Ogden and the sensorimotor tradition translate this neuroscience into clinical practice, arguing that trauma represents a failure or chronic dysregulation of these same responses, whose residue persists in the body as maladaptive action tendencies. Nijenhuis extends the framework further, proposing that somatoform dissociation is itself a form of defensive behavior—specifically the animal immobility response—triggered by overwhelming threat. Porges contributes the polyvagal perspective, introducing the hierarchical principle by which social engagement, mobilization, and immobilization defenses succeed one another phylogenetically. The field converges on the view that defensive behavior, when contextually appropriate, is adaptive; when chronically activated or fixated at a particular stage of the threat sequence, it becomes the substrate of psychopathology.