A computer system utilizing the most technologically sophisticated architecture, elegant user interfaces, and intuitive report generation capabilities, can be rendered nearly useless if the data it relies upon is fundamentally flawed. This chapter illustrates a variety of potential quality problems that could be present within the files and databases of any organization. The initial data propagation/transformation efforts can require significant revision and re-execution. Considerable additional effort and cost may be needed to repair data, to make it accessible, and to prevent continued corruption. The constraints defining what constitutes a “Valid” value for any given field should be clearly defined within each database or file, as it will be different in every case, and can even change for the same field over time. Referential integrity is a constraint to ensure that relationships between rows of data exist, and in particular that one row of data will exist if another does.