Bulk REBa2Cu3O7-? (REBCO, RE stands for Rare-Earth) superconductors find a large number of applications as levitation platforms, vibration dampers, fault current limiters, magnetic shields, microwave cavities and many more. It is necessary for such applications that the material is processed into required shapes and possesses microstructures that support high current densities (Jc) up to large applied magnetic fields. Details of the development of a novel process called Preform Optimized Infiltration Growth Process (POIGP) are discussed. POIGP enables fabrication of uniformly high Jc REBCO and REBCO/Ag superconducting composites with good homogeneity in the distribution of RE2BaCuO5 inclusions in the REBa2Cu3O7-? matrix. The work described in this book points to the nature and scale of defects required for improving the field dependence of Jc in REBCO products. The defect structure observed in the matrix of YBa2Cu3O7-? includes widespread, nano-sized, twin variants. Crossing nano-twins are observed when Y2BaCuO5 inclusions are in close proximity in the YBa2Cu3O7-?. The observed defect spacing and densities account for the uniformly high current densities observed to high fields.