Preface.- Analysis of input-affine nonlinear processes.- Basic Definitions of Differential Algebras.- Algebraic Observability Condition for Nonlinear systems and External behaviour.- Generalized Observability Canonical Forms.- Observer Synthesis.- Tracking and Stabilization Problems.- Parametric and State Estimation.- Observer synthesis for a more general class of Nonlinear Systems.- A Separation Principle for Nonlinear Systems.- Some uncommon observers with interesting applications.- Appendix A Singularity Treatment.- Appendix B Some properties for Nonlinear Systems.