Epigenetics is changing the way we think about disease. It helps explain how the same DNA can lead to very different outcomes, and how inflammation, metabolism, environmental exposures, and lifestyle can leave lasting effects on gene activity without altering the genetic code. This edited volume offers a concise, connected overview of key epigenetic processes such as DNA methylation, histone modifications, chromatin remodeling, and non-coding RNAs, and shows how they shape immune imbalance, tissue remodeling, vascular and organ aging, host–pathogen interactions, and cancer behavior, including progression and treatment response. Rather than treating each condition in isolation, the book highlights shared regulatory principles and the “rules” that govern epigenetic plasticity across systems. The result is a practical guide to the language of chromatin in health and disease, with clear value for researchers, clinicians, and advanced students seeking a reliable framework for interpreting complex phenotypes and understanding why epigenetic mechanisms have become central to biomarker development, patient stratification, and epigenome-informed therapeutic strategies.