A solution to the approximate tracking problem for networked control systems (NCSs) with uncertain, time-varying sampling intervals and network delays is presented. The uncertain, time-varying sampling and delays cause inexact feedforward, which induces a perturbation on the tracking error dynamics. Two alternative modeling approaches are used: a discrete-time model and a model in terms of delay impulsive differential equations. Sufficient conditions for the input-to-state stability (ISS) of the tracking error dynamics with respect to this perturbation are given. These ISS results provide bounds on the steady-state tracking error as a function of the plant properties, the controller parameters, and the network properties. The results are illustrated on a mechanical motion control example.