Renin, the first enzyme in the renin-angiotensin system, is critically important for the maintenance of blood pressure, and, therefore, as a target for antihypertensive therapy. The three-dimensional structure of renin would be an invaluable aid in understanding the functional properties of renin as well as in the design of novel, potent inhibitors. Three-dimensional models of renin have been developed by a number of different groups based on comparative homology modeling from the other known aspartic proteinase structures. These models have been used widely in the drug design process to suggest targets for synthesis and to rationalize the structure-activity relationships of compounds. This review describes the different published renin models and compares them to the extent possible. Applications of these model renin and renin-inhibitor complex structures to biological function and inhibitor design are summarized.