Most real system models are nonlinear, and many of them cannot be linearized and presented by linear state-variable equations. The nonlinear system models we are interested in are models of ground, mobile, and space manipulators, ground car-like vehicles, underwater vehicles, biomechanical or biomimetic systems, and many others. Their dynamics properties are governed by nonlinear ordinary differential equations. For some of these system models, kinematics relations may be used as a basis for design of control algorithms. These kinematics relations are discussed in Section 3.4. For some of them, only dynamics models may be used to design control algorithms; such systems are discussed in Section 3.5.