Post-traumatic stress disorder and acute stress disorder are mental health conditions with a known onset, and prevention strategies can therefore be used to try to prevent the emergence of the full-blown disorder. This chapter provides an overview of the current evidence-based prevention strategies for post-traumatic stress disorder and acute stress disorder. In the first part, diagnostic and epidemiological features of these disorders are considered. The second part of the chapter reviews the evidence base of current preventive psychological and pharmacological interventions. Although some early trials on primary intervention have shown promising effects, it appears too soon to provide any definite recommendation in clinical practice. Importantly, many current widely disseminated treatments lack evidence, and some interventions (for example, debriefing) may, in fact, have a negative impact on the natural recovery after trauma. This chapter highlights the importance of using science-driven interventions to prevent post-traumatic stress disorder and acute stress disorder.