HCV is a small positive-strand RNA virus responsible for a considerable proportion of acute and chronic hepatitis in humans. Although all HCV enzymes are, in theory, equally appropriate for therapeutic intervention, the NS3-NS4A serine protease and the NS5B RNA-dependent RNA polymerase are the most popular targets from a drug-discovery perspective. A number of active-site inhibitors of the NS3 protease as well as allosteric inhibitors of the NS5B polymerase are being developed. We determined the crystal structures of complexes of NS3/NS4A/active-site inhibitor as well as NS5B/allosteric inhibitor to permit structure-based drug design and the efficient optimization of leads. The methods for obtaining such structures by crystal soaking procedures are described.