Network-attached storage (NAS) systems are generally computing-storage devices that can be accessed over a computer network, rather than directly being connected to the computer (via a computer bus). This enables multiple users to share the same storage space at once, and often minimises overhead by centrally managing hard disks.

NASes usually contain one or more hard disks, often arranged into logical, redundant storage containers or RAID arrays.

NAS is an alternative to a specialized storage area network (SAN), where devices communicate using the block based SCSI protocol over dedicated Fibre Channel or Internet Protocol (IP) network. NAS devices are often connected to a shared general purpose local area network, reducing cost overhead, while also reducing access speed. The protocol used with NAS is a file based protocol such as Network File System (NFS) or Microsoft's Common Internet File System (CIFS).

NAS was pioneered by Auspex, based on the success of file servers from Novell, IBM, and Microsoft. The widespread success of NAS was due to the introduction of the Network Appliance Filer, and NAS market share in enterprise storage is increasing due to low total cost of ownership compared to Fibre Channel SANs. In recent years, emerging companies like BlueArc have combined the ease of management of NAS with the performance and scalability of SANs.

NAS was developed to address problems with direct attached storage, which included the effort required to administer and maintain "server farms", and the lack of scalability, reliability, availability, and performance. They can deliver significant ease of use, provide heterogeneous data sharing and enable organisations to automate and simplify their data management.