The course gives a general introduction to modern nonlinear control theory (with an emphasis on the differential geometric approach), as well as providing students specializing in nonlinear control theory with a firm starting point for doing research in this area. One of the authors' primary objectives is to give a self-contained treatment of all the topics covered. Although the amount of work published on nonlinear geometric control theory is expanding rapidly expanding, the authors confine themselves to treating solid and clear-cut achievements of modern nonlinear control, which can be expected to be of remaining interest. The final selection of topics reflects the authors' own judgement of their importance.