A food that is undergoing a fermentation process is a living system that is sheltering a multitude of coexisting microbial and enzymatic activities. The results of these activities are mainly catabolic processes, although the anabolic ones and their contribution cannot be undervalued. As a consequence of these activities, an almost limitless quantity of compounds is formed from the food components (carbohydrates, proteins, and lipids). These compounds have an unequal contribution to the avor and aroma of the fermented products, depending on their concentrations and perception thresholds that, in turn, depend on the chemical structures and properties of the molecules. Moreover, these compounds interact with each other, producing synergic (reinforcing) and masking phenomena.