Abstract: Comorbidity,being of significance for clinical decision,prognosis,and disease management,is one of the urgent health problems for chronic disease patients and older adults.In the article,the concept,types of comorbidity and the differences and connections between comorbidity and co-existence of multiple diseases were discussed,and the influences of comorbidity on mortality,survival years,and health service utilization were summarized.In addition,commonly used comorbidity indexes were reviewed and commented.The problems to be solved in future research concerning on comorbidity were pointed out.The present review provides basic information for the study on comorbidity and comorbidity index.