Abstract: Biological molecular motors will play a major role in fundamental science in the 21st century as a bridge between life science and nanotechnology. Though the investigation of these motors has been going on for over 150 years, significant breakthroughs only appeared in the last 20 years, thanks to the interdisciplinary studies of physicists, biochemists, physicians and mathematicians, and the development of single molecule technology. We review the major achievements in this field as well as current problems.