This chapter reviews the issues of adaptation and psychological disorders. First, the definitions of adaptation and psychological disorders are stated, and the criteria for psychological disorders are given. Depression is introduced as a common and typical example of psychological disorder. You can check your depressive tendency by the self-rating scale presented here. Second, after discussing an example of a person with depression, the biological (neurotransmitter) and psychological mechanisms of depression are explained. Theory of learned helplessness and attribution theory reveal the mechanisms of depression. You can check your attribution type and cognitive style and recognize your susceptibility to depression by following the instruction given here. Third, antidepressant medication and cognitive behavioral therapy (CBT) are each considered as an effective means for the treatment of depression. Fourth, historically famous persons who suffered from depression are introduced, and the risk of suicide in depressive patients is discussed.