Queueing network models have been extensively applied to represent and analyze resource sharing systems such as communication and computer systems and they have proved to be a powerful and versatile tool for system performance evaluation and prediction. Product form queueing networks have a simple closed form expression of the stationary state distribution that allow to define efficient algorithms to evaluate average performance measures. We introduce product form queueing networks and some interesting properties including the arrival theorem, exact aggregation and insensitivity. Various special models of product form queueing networks allow to represent particular system features such as state-dependent routing, negative customers, batch arrivals and departures and finite capacity queues.