Oxidized lipids are generated from (poly)unsaturated diacyl- and alk(en)ylacyl glycerophospholipids under conditions of oxidative stress. The great variety of reaction products is defined by the degree of modification, hydrophobicity, chemical reactivity, physical properties and biological activity. The biological activities of these compounds may depend on both, the recognition of the particular molecular structures by specific receptors and on the unspecific physical and chemical effects on their target systems (membranes, proteins). In this review, we aim at highlighting the molecular features that are essential for the understanding of the biological actions of pure oxidized phospholipids. Firstly, their chemical structures are described as a basis for an understanding of their physical and (bio)chemical properties in membrane- and protein-bound form. Secondly, the biological activities of oxidized phospholipids are discussed in terms of their unspecific effects on the membrane level as well as their potential interactions with specific targets (receptors) affecting a large set of (signaling) molecules. Finally, the role of oxidized phospholipids as important mediators in pathophysiology is discussed with emphasis on atherosclerosis.