Killer whales are among the most contaminated animals on the planet, accumulating high tissue levels of synthetic contaminants. These contaminants, including PCBs (polychlorinated biphenyls) and DDTs (dichlorodiphenyltrichloroethanes) have been banned for decades, but are very slow to degrade and show strong accumulation through the food web, leading to elevated exposures in top predators. Such exposures put killer whales, the ocean's top predator, at risk for endocrine, reproductive, and immune-related health effects. It has been shown that diet is an important factor in contaminant variation among killer whale groups. However, the diet within and among groups of killer whales in the North Atlantic is not well understood. Our project aims to use high-resolution chemical tracers of diet measured from biopsy samples of free-ranging North Atlantic killer whales to gain insight into different diets of major groups of North Atlantic killer whales, and in turn, to understand how this feeding variation may lead to differences in exposure to major contaminant classes of environmental concern.