Backstepping designs for nonlinear strict feedback systems (SFSs) are exquisite and ingenious. The trick in tying the virtual controllers of all layers together to guarantee the stability of the designed system is extremely wonderful and magnificent. In Chapter 9, it is shown that the idea and technology are extendable to second-order SFSs and mixed-order SFSs. Specifically, the robust high-order backstepping designs for both the second-order and the mixed-order SFSs with dynamical uncertainties are proposed, and so are the adaptive high-order backstepping designs for both the second-order and the mixed-order SFSs with an unknown parameter vector. They clearly possess the following advantages: The high-order backstepping designs are due to the high-order FAS approach. They cannot be produced within the state-space approach framework.