Secular trends in health outcomes related to dietary quality have changed substantially over the past three decades. The objective of this study is to decompose changes in dietary quality over 1977-2008 into the biological effects of age, contemporaneous effects of time period and group membership effects related to birth cohort. Dietary quality is measured by the Healthy Eating Index-2005 (HEI-2005). Using an Age-Period-Cohort (APC) model, findings suggest HEI-2005 scores follow a U-shaped trajectory over the typical American's lifetime (aged 2-79 y), with the lowest scores observed during early-to-mid adulthood (20-50 y, HEI = 54). Period effects largely increase throughout the sample period over 1977-2008. By examining over 100 birth cohorts (1898-2005), results show that those born in the first half of the 20th century have similar levels of dietary quality. However, following the end of the second World War, a substantial decrease in HEI-2005 scores is observed for those born between 1950 and 1990. Although recent cohorts in the 1990's and early 2000's have yet to fully age, preliminary estimates suggest dietary quality may be on the rebound. While nutritional and epidemiological studies tend to focus on secular trends (or period effects) this study suggests cohorts play an important role in characterizing changes in dietary quality.