Descriptor system is mixed of differential and algebraic equation which has a large number of applications. This class of systems may be classified as implicit, semi implicit and may also be called singular system. This book deals with solvability and some qualitative properties like stability, consistency, regularity and stabilization for non-linear descriptor systems with open(closed) loop feedback controller. A non-classical variational approach was developed for solving linear time varying(invariant) descriptor systems. Logarthmic norm approach has modified for some class of this system to ensure the stabilizability of some sense.