Squid is a high-performance proxy-caching server for web clients that can be arranged hierarchically for an improvement in response times and a reduction in bandwidth usage. It is free and open source software and supports FTP, gopher, and HTTP data objects. Squid is licensed under the terms of the GNU General Public License.

Unlike traditional caching software, Squid handles all requests in a single, non-blocking, I/O-driven process. It mainly runs on all Unix systems.

Following are some of the key features supported by Squid:
· proxying and caching of HTTP, FTP, and other URLs
· proxying for SSL
· cache hierarchies
· ICP, HTCP, CARP, Cache Digests
· transparent caching
· WCCP (Squid v2.3 and above)
· extensive access controls
· HTTP server acceleration
· SNMP
· caching of DNS lookups

Squid consists of a main server program squid, a Domain Name System lookup program dnsserver, some optional programs for rewriting requests and performing authentication, and some management and client tools.

Squid is derived from the ARPA-funded Harvest project. Squid is the result of efforts by numerous individuals from the Internet community. Duane Wessels of the National Laboratory for Applied Network Research leads code development.

Squid is copyrighted by the University of California, San Diego. However, Squid also uses some code developed by others.