Comorbidity is common in the older population. A variety of assessment techniques have been used for measurement of comorbidity and have demonstrated the association of increased level of comorbidity with a variety of adverse health outcomes. The most basic measure of comorbidity is a sum of the number of conditions present. Two general approaches have been used to incorporate disease severity measures into comorbidity indices: assignment of severity scores to all relevant conditions with summation of these scores, and assignment of comorbidity severity based on the most severe comorbid disease present. Each of these approaches has strengths and limitations. Research focusing on an index condition and associated comorbidity has been valuable in describing the impact of comorbidity in a number of different disease states.