The purpose of a traceability matrix is to show where in the life cycle a user requirement can be met either by testing (directly or indirectly) or by verification (e.g. delivery of a document or by a documented action). In this chapter we will look at what a traceability matrix is and its role in a CDS validation project and when this document should be written. From my perspective, a traceability matrix is an essential tool to help you control the detailed steps of any computer validation project, including a CDS.