Life‐threatening experiences such as car accidents, assaults, or natural disasters usually produce powerful and intrusive memories. In vulnerable individuals, these overly strong memories may persist and lead to the debilitating condition of posttraumatic stress disorder (PTSD). This chapter first focuses on the impact of stress and stress hormones on human episodic memory. It then reviews stress effects on human fear conditioning, an important model of PTSD, where fear memories can be established without explicit awareness of the learning process. The chapter summarizes recent findings showing that stress may alter the contributions of multiple memory systems to learning, thus promoting a shift from flexible, “cognitive” to rather rigid, “habit” learning and memory. Finally, the chapter discusses the implications of these findings on stress and memory in healthy humans for trauma memory, the core feature of PTSD.