It has been shown in recent studies that flavor and food-borne process contaminants are generated through similar reaction pathways, might it be the Maillard reaction cascade or oxidative degradation of lipids, polyphenols and other food constituents.While studies carried out in model systems based on simple precursors are valuable, they need to be validated in food under relevant process condition.The use of labelled precursors has allowed gaining a more precise insight into the formation pathways and estimating their relative importance.Since several years, such labelling experiments turned out to be a powerful tool to gain a deeper insight regarding the impact of reaction conditions on the formation of key flavor molecules and undesirable compounds in both model systems and real food to identify parameters that are essential for product quality.The approach will be illustrated by looking at the formation of certain aroma compounds (e.g.2,5-dimethyl-4-hydroxy-3(2H)-furanone) as well as process contaminants (e.g.acrylamide, furan) in thermally processed foods with focus on extruded cereals and cooked vegetables.