This article was written to reflect the importance of a correct quantification of the complexity of the elderly patient and its potential possibilities in clinical practice. From the decision to perform a laboratory test to the decision of the benefits of a determinate surgery through adjusting pathologies in several studies, most of our decisions depend on the complexity of our patients. Our decisions should be based on objective criteria that categorize thoe patients who may benefit from our decisions. One way is with comorbidity indexes. Another objective of this article was to review available methods to measure comorbidity and to assess its validity in the elderly. A search was made, with the keywords comorbidity, multi-morbidity and complexity associated with the elderly patient. The different methods were compared and different indexes are offered depending on our study’s patients.