This book provides an extensive introduction to the microstructures and structural defects in high-temperature superconductors. It illustrates the application of modern experimental techniques as well as theoretical modeling tools in the study of these complex materials.The readers are given an overview of the structure-sensitive properties, such as transport properties, and the effort to develop large-scale (high-current, high-field) applications for these materials. The effects of defects on the superconducting properties of these materials are described when feasible to put the study of microstructures in proper perspective.