Structural information on target proteins is used in molecular modeling to guide chemists and biologists in their daily work. For insight into the active centers of enzymes and receptors (e.g. GPCR, G-protein-coupled receptors), the three-dimensional structure must be determined either via X-ray crystallography or NMR. In structure-based molecular design, the three-dimensional structures of drug targets are used to guide drug discovery. The spatial visualization of protein-small-molecule complexes greatly facilitates the creation of ideas for new molecules. Automated positioning of small fragments in the active site of a target protein additionally speeds up the improvement of Structure Activity Relationship (SAR) as well as the design of new molecules with higher affinity or good pharmacokinetic properties (ADME). In the case of unavailability of the three-dimensional structures of a target protein, homology models can be built if suitable target proteins with high sequence identity are available.