The biological etiology of depression was recognized more than 30 years ago, and with the development of research techniques, great progress has been made to deepen the understanding of the etiology and pathophysiology of depression, the more prominent understanding is that the neurotic changes caused by certain substances in the body, such as: (1) the norepinephrine theory (NE): At that time, depression was thought to be caused by the lack or relative lack of catecholamines in the brain, especially NE deficiency was the cause. Some verification can be obtained from clinical treatment, however, there are still many problems with this single doctrine. (2) Pentraxin theory (5-HT): In recent years, this hypothesis has been increasingly studied, and there are many receptor blood subtypes related to emotional activity, and different receptor subtypes have different functions, which support the theory that depression is related to 5-HT dysfunction from a therapeutic point of view. The causes of depression are numerous. The main ones are adverse social and psychological factors. People live in a society with which their life span, health and disease are inextricably linked. Human worries, happiness, grief, relief, anxiety, etc., all emotions that maintain or disrupt normal human physiological functions are closely linked to society. Life conditions include changes in social, cultural and objective environment, changes in marriage, family, personal physical condition and interpersonal relationships, changes in economic conditions, personal academic success or failure, etc. Any conditions in life that can cause strong mental stress, severe trauma or unpleasant emotional experiences can become psychological factors. Many endocrine diseases, such as hyper- or hypothyroidism, Addison’s disease, Cushing’s disease, etc. can be accompanied by emotional symptoms, and various hormones can cause high or low moods, leading people to think that there may be a relationship between neuroendocrine and depression, and a lot of research data have been accumulated. In recent years, due to the development of biotechnology and the clinical application of electronic computer scanning (CT), magnetic resonance imaging (MRI), positron emission scanning (PET) and single photon emission computer scanning (SPECT), it has also been found that depressed patients may have altered brain function or brain structure. It has also been associated with genetic factors and personality traits. In conclusion, the etiology of depression is complex and is still under continuous research in countries around the world.