Older patients often suffer concomitant chronic diseases in addition to various health disorders. When attempting to study one of these diseases, it is necessary to consider the complexity of the patient's health status. Comorbidity is a cause of confusion, especially for elderly patients with multiple diseases. Comorbidity indexes have been developed in order to measure this complexity. We focused this article on a review of the literature in order to identify and examine the validity of comorbidity indexes applied to the elderly population. Five indexes have been used to measure comorbidity in patient populations with complex health situations: the Cumulative Illness Rating Scale (CIRS), the Kaplan-Feinstein index, the Charlson index, the Index of Coexistent Disease (ICED), and the Total Illness Burden Index (TIBI). Each of these indexes included a series of domains which vary according to the authors'view of comorbidity. These indexes were validated under different conditions with similar targets. Their validation fields limited their use and extrapolation of results. Only the CIRS, the Charlson index, the Kaplan-Feinstein index and the ICED were validated and applied to old patients. However, the Charlson index was found to be limited in recording the entirety of the old patients'pathologies, and in patients with cognitive deficits, only CIRS appeared to be sufficiently trustworthy because it allows a comprehensive recording of all the comorbid disease from clinical examination and medical file data. When studying chronic disease states in frail elderly patients, it is essential to consider comorbidity using standard validated indexes in order to get a comprehensive assessment of the patient's situation and avoid neglecting diseases and handicaps. Further studies are necessary to develop and validate tools specifically adapted to the elderly population.