Health issues are rarely limited to a single tissue or organ. A functional disorder that begins in a single organ cell first affects that organ and then spreads to all other organs, like a domino effect, leading to systemic deterioration of health. This cellular process manifests clinically as multiple diseases and/or comorbidities. Fundamentally, these disease burdens, which pose significant financial challenges for the healthcare system, rank among the foremost professional challenges for physicians. Although comorbidity, which is most commonly observed in later life, is often perceived as a health issue associated with aging. Multimorbidity, on the other hand, refers to health issues that can occur regardless of age. In this book, titled Understanding Multimorbidity and Comorbidity, each chapter addresses health issues and strategies for managing them.