Many important Management Support Systems require seamless access to and integration of multiple heterogeneous database systems. This paper studies heterogeneous database systems from the source perspective. It aims at addressing issues such as the following: (1) Where is the data from? (2) Which intermediate data sources were used to arrive at that data? Specifically, it presents a polygen model for resolving the Data Source Tagging and Intermediate Source Tagging problems. In addition, it presents the necessary and sufficient condition for source tagging. Source knowledge is important for many reasons. It enables users to apply their own judgment to the credibility of the information. It enables users to rationalize and reconcile data inconsistencies. It enables system designers to develop access charge systems. It enables an application user to adjust data. It enables a system to interpret data semantics more accurately. In sum, it justifies having source tagging capabilities as a required functionality for future heterogeneous database systems.