Defects and structural interfaces in superconducting Bi‐Sr‐Ca‐Cu‐O have been characterized by transmission electron microscopy. The superconducting phase exhibits frequent variations in the stacking sequence (polytypoids). Dislocations, observed inside the grains, either introduce or accommodate the shear in thea‐bplane and the local composition fluctuations. In general, the grains exhibit a platelike morphology with thea‐bplane as the grain boundary plane. Grain boundaries along the short edge are generally disordered, whereas those near the long edge generally have a thin layer of the lowerTcpolytypoid. Coherent intragranular boundaries are also observed.