Voronoia is a web tool which identifies cavities in a molecular structure and calculates the packing density of all atoms. This allows users to check for packing defects, which may be a sign of a wrongly resolved structure. The tool also helps in creating reliable simulations by finding cavities possibly missing internal water molecules in the structure, thereby enabling a more realistic modeling of proteins and nucleic acids. The results are visualized directly on the website with NGL. Voronoia also includes a database of precalculated structures from the Protein Data Bank which can be explored.