The SOLAR-A spacecraft was launched by the Institute of Space and Astronautical Science, Japan (ISS) in August 1991, and, following and ISAS tradition, was renamed YOHKOH. This mission is dedicated principally to the study of solar flares, especially of high-energy phenomena observed in the X-ray and gamma-ray ranges. With a co-ordinated set of instruments including hard X-ray and soft X-ray imaging telescopes as well as spectrometers with advanced capabilities, it is expected to reveal new aspects of flares and help better understand their physics, supporting international collaborations with ground-based observatories as well as theoretical investigations. An overview of this mission, including the stallite, its scientific instruments, and its operation is given in this book.