The GLP contains specific language concerning characterization of the test, control and reference substances used in toxicity studies. This paper will describe and discuss what types of documents are required to support test/reference substance characterization under GLP system. The purpose of this article is to present an overview of data needed in the characterization package that will adequately define the substance. Most sponsors use a certificate of analysis (COA) to communicate the test sub-stance characterization status information to the contracting research organizations. The COA should provide the test $material^{circ}{phi}s$ characterization results, substance storage requirements, expiration dates, verification of the collection of the retention sample, archival location of the data to support the characterization and GLP compliance status of the characterization.