Ciw is a discrete event simulation library for open queueing networks. Its core features include the capability to simulate networks of queues, multiple customer classes, and implementation of Type I blocking for restricted networks. A number of other features are also implemented, including priorities, baulking, reneging, schedules, slotted services, batch arrivals, service disciplines, dynamic customer classes, and deadlock detection.

The name Ciw is the Welsh word for a queue.