The problem of well-posedness has been discussed by several authors, G. Zanies and J. C. Willems in particular. Wo propose a new conceptual framework for the well-posedness of system models. In this framework well-posedness is viewed as the continuity of a specific property of a family of dynamical systems with'respect to a parameter change. This parameter change mny represent either errors due to modelling procedures, manufacturing tolerance, or tolerances involved in measuring the input data of the system. Several examples from different subfields of system theory are exhibited to illustrate the generality and flexibility of this new conceptual framework.