In this text experts review experimental studies thatdirectly reveal the relationship between the atomicstructure and physical behavior of high-Tc superconductors.The thorough discussion centers on twins, twin boundaries,the vortex lattice, and magnetic and mechanical propertiesin connection with structural imperfections. Particularattention is paid to the role of the oxygen atom in theY-Ba-Cu-O and La-Cu-O species. The experimental methodsevaluated include electron and X-ray diffraction, electronmicroscopy, and M|ssbauer spectroscopy.This book makes extraordinarily valuable data obtained atthe Institute of Solid State Physics at Chernogolovkaaccessible to the wider international community ofresearchers in superconductivity.