Packet writing is an optical disc recording technology used to allow write-once and rewritable CD and DVD media to be used in a similar manner to a floppy disk from within the operating system, i.e., it allows users to create, modify, and delete files and directories on demand without the need to burn a whole disc. Packet writing technology achieves this by writing data in incremental blocks rather than in a single block. The most common file system for packet writing systems is the UDF format.

Deleting files and directories of a CD-R using packet writing technology does not recover the space occupied by these objects but are simply marked as being deleted. Similarly, changes to files cause new instances to be created instead of replacing the original files. Because of this, the available space on a medium using packet writing technology will slowly diminish every time its content is modified. For rewritable discs, however, this doesn't necessarily occur; there exists technology that allows them to be used as truly removable storage media, fully and freely writable (see UDF).

Due to the characteristics of optical rewritable media such as CD-RWs and DVD-RWs, the ability of data sectors to hold their contents wear out by changing them frequently (since re-crystallized alloy de-crystallizes). To cope with this the packet writing system can remap bad sectors with good sectors as required. These bad sectors cannot be recovered by formatting the media.

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