Estimtion is the process of extracting information from data - data which can be used to infer the desired information and niay contain errors. Modern estimation methods use known relationships to compute the deslred information from the measurements. taking account of measurement errors, the effects of disturbances and control actions on the system, and prior knowledge of the information. Diverse measurements car! be blended to form "best" estimates, and information which is unavailable for measurement can be approximated in an optimal fashion. The intent of this book is to enable readers to achieve a level of competence that will permit their participation in the des~gn and evaluation of practical estimators. Therefore, the text is oriented to the applied rather than theoretical aspects of optimal estimation. It is our intent throughout to provide a simple and interesting picture of the central issues underlying modern estimation theory and practice. Heuristic, rather than theoretically elegant,arguments are used extensively, with emphasis on physical insights and key questlons of practical importance.