Using transmission electron microscopy techniques, we have begun a program to study cascade defects in the high-Tc superconductor YBa2Cu3O7-δ. In situ ion irradiations in the high-voltage electron microscope (HVEM) accelerator facility at Argonne National Laboratory were performed to simulate low-dose neutron irradiation at both room temperature and at 40 K. Diffraction contrast experiments performed in two-beam dark field conditions using different operating reflections indicated defect cascades with hydrostatic pressure-like strain fields. Visible cascade defect yields ranged from 0.3 to 1.0, depending on incident ion mass. High-resolution micrographs revealed that at least some of these cascades have recrystallized regions that have rotated with respect to the surrounding lattice. Samples which were neutron irradiated to doses sufficient to produce overlapped cascades show a “cellular” structure of good crystalline material surrounded by highly defective or amorphous cell walls. It is suggested that the recrystallization observed in isolated cascade defects produced by low-dose ion irradiation provides the mechanism for the formation of the cellular structure at high damage levels. We will suggest correlations between neutron irradiation responses of the microstructure and the critical current density measured magnetically and also electrical resistivity measurements of the superconducting to insulating transition.