Patients with a complex history of illness and multiple diseases are increasingly the rule rather than the exception. To describe these patients, the term comorbidity is frequently used. The present overview provides a definition and description of the construct comorbidity. Thereby, the overview focuses on specifics of somatic-psychiatric comorbidities. The description of comorbidity comprises definition of the term against the similar constructs multimorbidity, burden of illness, and patient complexity as well as a subcategorization of types of comorbidity. With regard to measuring comorbidity, possibilities of aggregating comorbidities into a single sum score are presented. Finally, specific aspects of somatic-psychiatric comorbidity are critically discussed.