The convergence of greatly enhanced technologies for the detection of X-rays in single-crystal diffraction experiments created a renewed interest in the crystal chemistry of U6+, and resulted in an explosion of new structures containing U6+. During the past decade, known structures of uranyl compounds have more than doubled. These studies in this book have further revealed the tremendous structural elegance, beauty, and complexity of uranyl compounds. Unlike U6+, relatively few crystal structures have been analyzed that contain low-valence uranium. Considerable efforts have been made in synthesizing polynuclear low-valence uranium cluster structures, due to their potential applications in nuclear industry and natural environment as well as in the field of magnetic properties and catalysis.