We consider factors that limit the loss‐free conduction of electric current in the vortex state of high‐temperature superconductors (HTS). Simple arguments lead to an estimate of the maximum possible critical current densities arising from optimally‐sized linear defects that pin each vortex core over its entire length. In a series of model experiments, such linear defect structures have been tailored at controlled densities in several HTS materials by irradiation with high‐energy heavy ions. Results from these studies are compared with the estimated zero‐temperature theoretical limits. At higher temperatures, the irradiated materials provide unique systems for evaluation of the current theories of flux motion that account for both the intrinsic, anisotropic material properties and the unique defect structures. In particular, from measurements that relate the pinning to the angle between the vortex and defect lines, direct information can be obtained about the influence of material anisotropy on the vortex structure. Results will be compared for different classes of HTS materials.