Exposure to stressful situations occurs throughout an organism’s life. Stress can be defined as events that threaten an individual, triggering physiological and behavioral responses as part of the allostatic process. Thus, several physiological changes are necessary for the organism to deal with these events properly. Under certain conditions, the same mechanisms that generate stress responses can cause or enhance pathological conditions. Indeed, mental disorders, such as anxiety and stress disorders, like post-traumatic stress disorder (PTSD), are examples of pathological conditions that can arise from exhaustion generated by a stressful event. This chapter discusses the importance of molecular and physiological responses to stressful stimuli and how different stressors can cause different responses, especially in neural circuits that govern anxiety-like behavior. It also provides an overview of biomarkers associated with the harmful effects of stress.