This Chapter 2 concentrates on the FAS models. Firstly, starting with modeling of a second-order physical mass-spring-damper system and a first-order attitude system of spacecraft, this chapter introduces the single-order global FASs. Differences and relations between physical FASs and the proposed generalized FASs are discussed. Secondly, by investigating a motivating example, the more general models of multi-order FASs including affine multi-order FASs and nonaffine multi-order FASs are introduced. Thirdly, as a typical example, linear systems are treated with the FAS approach. It is shown that both controllable LTI and lexicography controllable linear time-varying systems can be equivalently converted into linear FASs, which permits immediately extremely simple strategies for eigenstructure assignment and stabilization of linear systems. Last but not the least, the first type of nonlinear systems, namely, globally feedback linearizable systems, are investigated. Specifically, it is shown that an affine and a nonaffine nonlinear system is feedback linearizable if and only if it can be converted into a global FAS.