Book excerpt: The past decade has witnessed an increasing interest in observers for nonlinear systems. This subject is relevant in different contexts such as synchronization of complex dynamical systems, fault detection and isolation, and output feedback control. This book contains the contributions that are to be presented at the workshop "New Directions in Nonlinear Observer Design", to be 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 will include participants from Africa, Asia, Europe and USA and it will focus on recent developments in the above mentioned areas. The contributions form a good 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.