HDL particles possess important antiatherogenic functionalities and understanding of the molecular mechanisms underlying these effects requires detailed knowledge of HDL structure. This review summarizes current understanding of HDL structure. The various HDL subclasses are compared in terms of their lipid and protein compositions. The lipid-binding properties of the principal HDL apolipoprotein, apo A-I, permit plasticity in HDL structure. The amphipathic alpha-helical domains that are the major element of secondary structure mediate the interaction of apo A-I with phospholipid. Low resolution models of the structures of both discoidal and spherical HDL particles are evaluated. HDL particles are dynamic in that they are being remodeled constantly in vivo by interaction with lipases, lipid transfer proteins, and cell-surface HDL receptors. Current knowledge of the ways in which HDL particle structure is modulated by interactions with proteins such as LCAT, CETP, SR-BI and ABCA1 is reviewed.