This chapter describes two of the frequently occurring and important peritraumatic responses, namely, peritraumatic dissociation (PD) and tonic immobility (TI). It focuses on the definition of each phenomenon and their associations with posttraumatic psychopathology as it considers the relevant neurobiology. Dissociative reactions that may occur during trauma exposure include emotional numbing or detachment, reduced awareness, and distortions of reality. The main feature of TI is reversible physical immobility and muscular rigidity, which can last from a few seconds to many hours. Research regarding the basis, function, and mechanisms underlying the TI response has resulted in the acceptance of the fear hypothesis (FH), a multidimensional model of TI. Researchers have also examined the brain structures involved in the expression of TI, and three regions appear to be the most relevant to the induction and inhibition of this phenomenon: the frontal lobes, the limbic system, and the brainstem.