Geriatric health care settings represent an optimal place for mental health professionals to practice. The term "comorbidity" refers to the co-occurrence of diseases. In this chapter, we will review the comorbidity of medical and psychosocial problems-that is, whether psychosocial problems are common in medical rehabilitation patients--and whether there are enough psychosocial problems in geriatric medical patients for them to be given treatment consideration by hospitals. A second type of comorbidity, that of the co-occurrence of two or more psychosocial problems, will also be explored later in this chapter. Comorbid problems such as dementia, delirium, depression, and substance abuse and their occurrence in older medical patients were recently studied by our research group.