The most common reason for consumer rejection of a food product is an unacceptable flavor. Food flavor may become unacceptable due to many reasons. For example, the food may become contaminated by airborne-, waterborne-, or packaging-related chemicals. Alternatively, food components may undergo degradation due to oxidation, nonenzymatic browning, enzymatic reactions, or light-induced reactions. This review will provide a general description of the sensory properties, mechanism of occurrence, and recent research developments for the commonly occurring off-flavors in foods.