Shape TK is the basis for the ROCS application for shape-based similarity searching. The toolkit allows for fine control over optimization methods, molecule treatment and the nature of the query (grids, generic shapes). Shape TK facilitates the calculation of molecular descriptors for shape (steric multipoles), volume overlap between molecules, and spatial similarity of chemical groups (color force field), as well as the optimization of the latter two quantities. Examples of use include aligning and comparing protein active sites, performing real-space fitting to electron density, shape fingerprinting [1], consensus shape generation, and discrimination between agonists and antagonists based on shared shapes [2]