The CYCLADES computer network (French pronunciation: [siklad]) was a French research network created in the early 1970s. It was one of the pioneering networks experimenting with the concept of packet switching and, unlike the ARPANET, was explicitly designed to facilitate internetworking.

The CYCLADES network was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself. Datagrams were exchanged on the network using transport protocols that do not guarantee reliable delivery, but only attempt best-effort. To empower the network leaves (the hosts) to perform error-correction, the network ensured end-to-end protocol transparency, a concept later to be known as the end-to-end principle. This simplified network design, reduced network latency, and reduced the opportunities for single point failures. The experience with these concepts led to the design of key features of the Internet Protocol in the ARPANET project.

The network was sponsored by the French government, through the Institut de Recherche en lnformatique et en Automatique (IRIA), the national research laboratory for computer science in France, now known as Inria, which served as the co-ordinating agency. Several French computer manufacturers, research institutes and universities contributed to the effort. CYCLADES was designed and directed by Louis Pouzin.

Planning for the project began in 1971. Design and staffing started in 1972, and November 1973 saw the first demonstration, using three hosts and one packet switch.

Pouzin coined the term catenet in 1973, in a note circulated to the International Network Working Group, to describe a system of packet-switched communication networks interconnected via gateways. He later published these ideas in a 1974 paper "A Proposal for Interconnecting Packet Switching Networks".

Deployment of the network continued in 1974, with three packet switches installed by February, although at that point the network was only operational for three hours each day. By June the network was up to seven switches, and was available throughout the day for experimental use.

A terminal concentrator was also developed that year, since time-sharing was still a prevalent mode of computer use. In 1975, the network shrank slightly due to budgetary constraints, but the setback was only temporary. At that point, the network provided remote login, remote batch and file transfer user application services.

By 1976 the network was in full deployment, eventually numbering 20 nodes with connections to NPL in London, ESA in Rome, and to the European Informatics Network (EIN).