This open access book is written by world-recognized experts in the fields of applied superconductivity and superconducting accelerator magnet technology. It provides a contemporary review and assessment of the experience in research and development of high-field accelerator dipole magnets based on Nb3Sn superconductor over the past three decades. The reader attains clear insight into the development of Nb3Sn composite superconducting wires and Rutherford cables, and details of accelerator dipole design, fabrication, testing, and data analysis. Special attention is given to innovative features of the developed Nb3Sn magnet designs and technologies. The book concludes by a discussion of the main open areas of R&D in this field.