An understanding of the mechanisms of personality development provides a systematic way to promote health as an integrated state of physical, mental, social, and spiritual well-being. Individual differences in personality are causal antecedents of the full range of psychopathology. The maturation with integration of personality appears to be an important mechanism by which diverse modalities of treatment promote wellness and reduce illness. First, the authors review the relationship between personality and a wide range of psychiatric disorders. Second, the authors evaluate the impact of character structure on a wide range of measures of well-being, including positive emotions, negative emotions, life satisfaction, perceived social support, and perceived health. Third, the authors describe a practical and inexpensive clinical method for facilitating the maturation and integration of personality based on an understanding of the processes of human thought, which underlie changes in personality and well-being.