There are many systems which fail to satisfy the stationary assumptions. These systems contain some on-stationary events and are explicitly dependent on time. These type of systems are known as time varying systems. The authors cannot use time invariant models to characterize these type of systems. They require time-varying parametric models like Time-Varying Auto-RegreSsive with an exogenous (TVARX) or Time Varying Auto-Regressive (TVAR) models for the modeling and analysis of time varying systems. Method of parameter estimation is known as Adaptive recursive estimation. Adaptive recursive estimation method is called as stochastic approach, where coefficients of the associated model are treated as random processes with some stochastic model structure. Adaptive algorithms used in this approach are LMS algorithm, RLS algorithm and Kalman filter algorithm.