Both bipolar disorder and depression belong to a large group of disorders: called mood disorders or affective disorders. They can be understood as mood disorders, i.e., a group of disorders related to a person’s emotions. Among this large group of mood disorders or emotional disorders, the two most common disorders are depression and bipolar disorder. Depression is also called monophasic, and monophasic disorder refers to depression. As the name implies, it means that each episode of the illness has only one clinical phase: a depressed mood. By biphasic, I mean that there are episodes that manifest as low mood and depression. There are episodes that manifest as heightened mood. Depending on the degree and magnitude of the elevation, it is classified as a hypomanic or manic episode. The traditional name is manic-depressive psychosis, but now it is called bipolar disorder or bipolar disorder. In most bipolar patients, 70% of the episodes are depressive. The period of elevated mood is maintained for a very short period of time. Both the number of episodes and the duration of the episodes are much lower than the cumulative duration of the depressive episodes, so it is easy to misdiagnose these two disorders. The overall treatment principles for depression and bipolar disorder are fundamentally different. If a patient with bipolar disorder is diagnosed with depression and treated according to depression, there is a risk that his condition will become less and less stable. It may even be induced, and the depression may be induced into a light manic or manic episode. How to differentiate these two diseases is a worldwide problem. Bipolar is also a highly prevalent disorder.