Network-attached storage (NAS) is file-level computer data storage connected to a computer network providing data access to network clients.

A NAS unit is essentially a self-contained computer connected to a network, with the sole purpose of supplying file-based data storage services to other devices on the network. The operating system and other software on the NAS unit provide data storage, file systems, access to files, and the management of these functionalities. The unit is not designed to carry out general-purpose computing tasks, although it may technically be possible to run other software on it. NAS units usually do not have a keyboard or display, and are controlled and configured over the network often by connecting a browser to their network address.

NAS systems contain one or more hard disks, often arranged into logical, redundant storage containers or RAID arrays (redundant arrays of inexpensive/independent disks). NAS removes the responsibility of file serving from other servers on the network. NAS uses file-based protocols such as NFS (popular on UNIX systems) or SMB (Server Message Block; used with MS Windows systems). NAS units rarely limit clients to a single protocol. NAS provides both storage and file system, as contrasted with a SAN (Storage Area Network) which provides only block-based storage and leaves file system concerns on the "client" side.