Have you ever had trouble finding an important item in your room because the room was cluttered and disorganized? Have you ever had trouble finding a file on your computer because you could not remember what it was named? Computing systems commonly manage billions of items with every passing moment. A computing system must keep precise account of this data as it flows through the system and this requires close attention to every small detail. In particular, this chapter explains that all data must be well organized and properly identified in order to be usable.