This talk examines the problems of unused, hidden or legacy data in software systems. For software that has evolved over many years, the purpose and intent behind the data items (e.g. constants in source files, initialisation data, and system configuration values) may be lost in the mists of time, with little or no supporting documentation. Sometimes this data is used within the system, sometimes not, and sometimes it should be used when it is not, and not used when it should be. This talk categorises types of Dead Data, Hidden Data and Change Data and explains how they are identified, named and managed. The naming and metaphorical imagery used is considered relevant, as it helps with identification and awareness in the minds of the developers charged with maintaining complex legacy software systems. There is an analogy with Dead and Deactivated code, a concept used in several standards and guidance including DO-178C.