Choosing models related effectively to the questions to be addressed is a central issue in the craft of systems analysis. Since the mathematical description the analyst chooses constrains the types of issues he candeal with, it is important for these models to be selected so as to yield limitations that are acceptable in view of the questions the systems analysis seeks to answer. 

In this paper, the author gives an overview of the central issues affecting the question of model choice. To this end, he discusses model components and a wide variety of possible mathematical system descriptions. After discussing both local and global aspects of these model types, headdresses basic questions and perspectives of system theory. The paper concludes with a sketch of a systematic response to the question: What model to choose? 

To provide a thorough overview of systems analysis, the International Institute for Applied Systems Analysis is preparing a "Handbook of Systems Analysis" in three volumes: Volume 1, "Overview;" Volume 2, "Methods;" Volume 3, "Cases." This essay is a contribution to the  second volume of this Handbook.