Diet quality is increasingly recognized as an essential determinant of health, partly due to its capacity to shape the epigenome via changes in DNA methylation. This chapter synthesizes evidence from epigenome-wide association studies that assessed the association between dietary changes and DNA methylation in adults and children. It highlights how dietary patterns—particularly adherence to healthy dietary indices and the intake of ultra-processed foods—may modulate methylation at specific position of the DNA that are, in turn, linked to inflammation, glucose regulation, lipid metabolism, and overall cardiometabolic risk. The chapter also addresses the current methodological challenges in diet-epigenetics research, including reliance on self-reported dietary data, potential tissue-specific effects, and the complexities of establishing causality. Lastly, it points to future research directions in order to develop better personalized nutrition strategies. By clarifying the epigenetic impact of dietary habits, these findings might inform more targeted public health and clinical interventions to mitigate chronic disease risk across the lifespan.