Xin Xin received the B.S. degree in 1987 from USTC, and the Ph.D. degree in 1993 from Southeast University. He is now a professor at the Okayama Prefectural University and is the Chair of the Department of Systems Engineering. He has more than 180 publications in journals, international conferences and book chapters. His current research interests include robotics, dynamics and control of nonlinear and complex systems. He is now associate editors of IEEE Control Systems Letters, Transactions of the Society of Instrument and Control Engineers, Journal of the Robotics Society of Japan. Underactuation is a technical term used in robotics and control to describe mechanical devices that have a lower number of actuators than degrees of freedom. The study of design and control for underactuated robotic systems (URSs) is a fertile research area having fruitful interactions with robotics and control, and has attracted many researchers from these two communities. This talk presents a unified treatment of control design and analysis for a class of URSs we studied for more than a decade, which includes systems with multiple-degree-of-freedom and/or with underactuation degree two. This talk presents some new notions, features, design techniques, and strictly global motion analysis results for these systems. The theoretical developments are validated by experimental results for several systems. It is expected that this talk will not only enable the listener to gain a better understanding of the power and fundamental limitations of linear and nonlinear control theory for the control design and analysis for these URSs, but also inspire the listener to address the challenging problems of more complex URSs.