Several contributing factors have been proposed for expression of excessive anxiety. Behavioral inhibition, activation of amygdala, and genetic vulnerability interchangeably affect each other. Only few studies have addressed the differences and similarities in anxiety between animal and human. The consensus is that individual vulnerability is a main factor for the expression of anxiety, although the interaction of environmental stressors and biological characteristics is involved in the presentation of anxiety. Further work is required to investigate how the interaction between stress and diathesis can affect responsiveness of stress and anxiety in animal and human. Here, the author reviews the findings regarding differences and similarities in both animal and human studies. Knowing the reciprocal relationship between the environment (stressors) and individual vulnerability or resilience, the proper understanding of anxiety and anxiety disorders would be possible.