It is well-documented that insecure attachment poses substantial risks to post-trauma recovery. Although attachment anxiety reliably predicts and is associated with elevated post-traumatic stress symptoms (PTSS), the attachment avoidance–PTSS link has been far less consistent. This suggests both attenuating and enhancing connections between attachment difficulties and post-trauma outcomes. The goal of this study is 2-fold: (a) to review and summarize extant evidence concerning the relation between insecure attachment and PTSS and (b) to review mechanism(s) underlying attachment anxiety and attachment avoidance that may explain the development of PTSS. Following a systematic review of the empirical literature investigating attachment, trauma, and post-trauma reactions, a total of 138 studies were identified and summarized. The review further identifies explanatory mechanisms underlying the association between insecure attachment and PTSS, including stress appraisals, cognitive factors, self-esteem, emotion regulation strategies, social factors, and trauma-specific factors (i.e., type of trauma, extreme stress). Implications for practice, policy, and research are discussed.