The data flow diagram has been extensively used to model the data transformation aspects of proposed systems. However, previous definitions of the data flow diagram have not provided a comprehensive way to represent the interaction between the timing and control aspects of a system and its data transformation behavior. An extension of the data flow diagram called the transformation schema is described. This schema provides a notation and formation rules for building a comprehensive system model, and a set of execution rules to allow prediction of the behavior over time of a system modeled in this way. The notation and formation rules allow depiction of a system as a network of potentially concurrent `centers of activity' (transformations), and of data repositories (stores), linked by communication paths (flows). The execution rules provide a qualitative prediction rather than a quantitative one, describing the acceptance of inputs and the production of outputs by the transformations but not input and output values.