Observers for nonlinear systems are relevant in different contexts such as synchronization of complex dynamical systems, fault detection and isolation, and output feedback control. These are the contributions presented at the workshop "New Directions in Nonlinear Observer Design", held from June 24-26, 1999, in Geiranger Fjord, Norway. The workshop has been organised by Olav Egeland, Thor I. Fossen and Henk Nijmeijer. It includes participants from Africa, Asia, Europe and USA and focuses on developments in the above-mentioned areas. The contributions form a review of present achievements and challenges in nonlinear observer design. The workshop is supported by the Strategic University Program on Marine Cybernetics at the Norwegian University of Science and Technology and ABB.