In this paper we show that the traditional methods of analyzing cascade control based on series or parallel input/output relationships should not be used when the primary and secondary processes are coupled through the state-space model and the process is open-loop unstable. The parallels between traditional full state feedback control and cascade control are demonstrated. We use a jacketed CSTR example to show that neither the parallel nor series input/output formulation for cascade control satisfies controllability or stabilizability conditions when the process is open-loop unstable, although these conditions are satisfied by the physical (state-space) model. We also present a cascade control design procedure which incorporates the "inner"-loop controller in the "outer"-loop design.