Preface. Acknowledgements. Elementary system and observer theory. Minimal-order state observers. Linear state function observers. Dynamical observer-based controllers. Minimum-time state reconstruction of discrete systems. Observers and linear least-squares estimation for stochastic systems. Adaptive observers. Observer-based compensation in the frequency-domain. Observer-based compensation for polynomial matrix system models. Geometric theory of observers. Further study. References. Appendix A and B. Author index. Subject index.