Multimorbidity, the coexistence of two or more chronic conditions, is prevalent among older adults, with 67% of Medicare beneficiaries in the U.S. having multiple chronic conditions. This prevalence increases with age, reaching 81.5% for those aged 85 years or older. A systematic review of 17 studies found a median prevalence of 63% and a mode of 67%. Variability in prevalence rates is likely due to methodological differences. Multimorbidity is associated with increased risks of death, disability, poor functional status, poor quality of life, and adverse drug events. Standardized methods for measuring multimorbidity are needed for public health surveillance and prevention. The review highlights the complexity of multimorbidity, with key methodological issues including the definition of chronic conditions, ascertainment methods, and source populations. Studies often rely on self-reported diagnoses or administrative claims data, which may lead to misclassification. The choice of how many chronic conditions to include significantly affects prevalence rates. Some authors suggest a minimum of 12 conditions, but the criteria for selection are not clearly defined. Multimorbidity is linked to various health outcomes, including increased mortality, functional impairment, and reduced quality of life. It is also associated with higher healthcare costs and the need for complex clinical care. The review emphasizes the importance of developing interventions targeting multimorbidity, including clinical and population-based strategies. Risk factors for multimorbidity include socioeconomic status, education level, and lifestyle factors such as smoking and diet. The leading causes of death related to multimorbidity are tobacco use, unhealthy dietary patterns, alcohol consumption, and physical inactivity. The review also discusses the implications of multimorbidity for policy and clinical practice, emphasizing the need for guidelines that address common comorbidities. The integration of multimorbidity into clinical care and public health strategies is crucial for improving health outcomes. In conclusion, defining multimorbidity and developing standardized measurement tools are essential for understanding and addressing the challenges posed by this condition. Future research should focus on causal pathways and the development of effective interventions to improve health outcomes for older adults with multimorbidity.Multimorbidity, the coexistence of two or more chronic conditions, is prevalent among older adults, with 67% of Medicare beneficiaries in the U.S. having multiple chronic conditions. This prevalence increases with age, reaching 81.5% for those aged 85 years or older. A systematic review of 17 studies found a median prevalence of 63% and a mode of 67%. Variability in prevalence rates is likely due to methodological differences. Multimorbidity is associated with increased risks of death, disability, poor functional status, poor quality of life, and adverse drug events. Standardized methods for measuring multimorbidity are needed for public health surveillance and prevention. The review highlights the complexity of multimorbidity, with key methodological issues including the definition of chronic conditions, ascertainment methods, and source populations. Studies often rely on self-reported diagnoses or administrative claims data, which may lead to misclassification. The choice of how many chronic conditions to include significantly affects prevalence rates. Some authors suggest a minimum of 12 conditions, but the criteria for selection are not clearly defined. Multimorbidity is linked to various health outcomes, including increased mortality, functional impairment, and reduced quality of life. It is also associated with higher healthcare costs and the need for complex clinical care. The review emphasizes the importance of developing interventions targeting multimorbidity, including clinical and population-based strategies. Risk factors for multimorbidity include socioeconomic status, education level, and lifestyle factors such as smoking and diet. The leading causes of death related to multimorbidity are tobacco use, unhealthy dietary patterns, alcohol consumption, and physical inactivity. The review also discusses the implications of multimorbidity for policy and clinical practice, emphasizing the need for guidelines that address common comorbidities. The integration of multimorbidity into clinical care and public health strategies is crucial for improving health outcomes. In conclusion, defining multimorbidity and developing standardized measurement tools are essential for understanding and addressing the challenges posed by this condition. Future research should focus on causal pathways and the development of effective interventions to improve health outcomes for older adults with multimorbidity.