In this chapter we present the singular perturbation method for continuous and discrete control systems. The boundary-layer method is also discussed where the approximate solution is given by the outer series and a boundary-layer correction which is equivalent to the difference between the inner and inter mediate series. The boundary-value problem is treated and the structural properties-stability, controllability and observability-are discussed. We conclude by bringing out various characteristic features of singular perturbations in differential and difference equations describing continuous and discrete control systems, respectively.