Polyphenols are a large family of plant-derived bioactive compounds widely present in the human diet, with growing evidence supporting their protective role against noncommunicable diseases (NCDs) such as cardiovascular disease, type 2 diabetes, and cancer. This chapter reviews epidemiological findings on polyphenol intake in Japan, drawing on comprehensive food composition databases and large-scale cohort studies. Green tea and coffee emerged as principal sources of polyphenols, contributing significantly to total intake. Studies in Japanese populations have linked higher polyphenol consumption with reduced risks of all-cause mortality, cardiovascular disease, and site-specific cancers, notably liver and colon cancer. However, methodological limitations, including reliance on dietary questionnaires and variability in polyphenol metabolism, underscore the need for biomarker-based validation. Recent advances in plasma polyphenol measurement support the feasibility and reliability of objective exposure assessment. Integrating dietary and biomarker approaches will be critical to deepening understanding of polyphenol-related disease prevention and informing evidence-based nutritional guidelines.