Malonyldialdehyde (malondialdehyde, MDA) is one of the most significant of the wide range of metabolic or deteriorative products of lipid damage. Its formation during food processing and storage is widely used in quality control. Its ability to react with -NH2 groups is also of great carcinogenic or mutagenic interest because of its possible interaction with important constituents (e.g. free amino acids, proteins, nucleic acids, and amino phospholipids) of foods during and after processing. Special attention has been given to the role played by MDA in the formation of the age pigments, commonly named as 'lipofuscin'.The present review compiles the general characteristics that have made MDA the most widely studied lipid metabolite, and the progress achieved in knowledge of MDA reactivity with biological molecules. The special stress given to the structural elucidation of the structures of these products have led to a new perspective about the role played by MDA.