The increasing performance demands on controlled chemical operations, coupled with the lack of appropriate measurement technology, motivates the need for nonlinear inferential control approaches. In this article, an overview of nonlinear inferential control approaches is presented,with an emphasis on the estimation component of the algorithm and logical extensions of the linear theory. Applications of the algorithms to process applications are reviewed, and new theoretical and application results are presented. (C) 1998 Elsevier Science Ltd. All rights reserved.