Clearly, besides reference signals, a controller algorithm needs measured plant outputs as inputs. Given a plant model, a question is if all internal system states or at least a subset of them are observable. The chapter focuses on complete state observability and recalls the criteria of Kalman, Hautus and Gilbert. If a system is not completely state observable, the Hautus criterion can be used to identify the non-stable system modes (the eigenvalues with a positive real par).