Low Jc values are typical for polycrystalline high temperature superconductors, especially in magnetic fields. On the other hand, recent results on some polycrystalline samples are showing encouraging Jc values. Grain boundary defects which limit current will be considered. The intrinsic properties of grain boundaries in the high Tc materials will be emphasized. These properties such as the atomic arrangements at grain boundaries are inherent to a given material. It will be shown that a plausible explanation for the low Jc values in c‐axis aligned YBa2Cu3O7compared to Bi2Sr2Ca1−xCuxOyis the ability to transfer supercurrents across basal plane boundaries in the latter material.