The centuries-old stigma attached to cancer precluded patients' being told their diagnoses, and thus, delayed any exploration of how they dealt with their illness. This situation changed in the United States in the 1970s when patients began to be told their cancer diagnosis, permitting the first formal study of the psychological impact of cancer. However, a second and equally long-held stigma attached to mental illness has been another barrier and this has kept patients from being willing to acknowledge their psychological problems and to seek counseling. This "double stigma" has slowed the development of psycho-oncology. However, we began to see rapid changes occurring in the last quarter of the 20th century. Valid assessment instruments were developed which were used in well-designed studies. Data from these studies and clinical observations led to increased recognition that psychosocial services are needed by many patients and provide significant assistance in coping with illness. Psycho-oncology has two dimensions: first, the study of the psychological reaction of patients at all stages of the disease, as well as of the family and oncology staff; second, exploring the psychological, social, and behavioral factors that impact on cancer risk and survival. Psycho-oncology now has a recognized role within the oncologic community through clinical care, research, and training as it relates to prevention of cancer through lifestyle changes, evaluation of quality of life, symptom control, palliative care and survivorship. Presently, there are sufficient research studies from which standards of care have been established. Both evidence and consensus-based clinical practice guidelines have been promulgated. It now possible to monitor the quality of existing psychosocial services by using these benchmarks of quality that have evolved in recent years.