In structure-based drug design, the three-dimensional structure of a drug target interacting with small molecules is used to guide drug discovery. "Structure-based drug design represents the idea that you can see exactly how your molecule interacts with its target protein.”This structural information can be obtained with X-ray crystallography or nuclear magnetic resonance spectroscopy (NMR). Originally, structure-based drug design was equated with de novo design or building a molecule from the ground up. Structure-based drug design relies on knowledge of the three dimensional structure of the biological target obtained through methods such as x-ray crystallography or NMR spectroscopy. If an experimental structure of a target is not available, it may be possible to create a homology model of the target based on the experimental structure of a related protein. As experimental methods such as X-ray crystallography and NMR develop, the amount of information concerning 3D structures of biomolecular targets has increased dramatically. Current methods for structure-based drug design can be divided roughly into two categories : Ligand-based drug design & receptor-based drug design.