In tracing the evolution of computer based Management Support Systems (MSS), three main generations or classes can be distinguished. These generations, in order of their emergence, may be identified as Data Management Systems (DMS), Information Management Systems IMS), and Knowledge Management Systems (KMS). The common objective among these three classes of systems is supporting managerial decision making. One distinguishing aspect among these three classes is the type of input and output (viz. data, information, and knowledge) that these systems handle, and consequently the level and the form of support that they provide. In this paper we explore these three classes of systems from an input-output perspective. In particular, this paper offers a framework for classifying MSS based on their input and output types, with particular focus on the input side. By developing an input-output based framework for current MSS, we hope to provide a foundation to develop an agenda for the next generation of MSS.