The existing simulators cannot provide an environment for simulating various protocols related to the ATM networks. Setting up an ATM network, which involves fiber optic cable and switches, is costly. The performance analysis of the ATM networks for different kinds of resource reservation and congestion control schemes, is required to decide whether to implement a particular scheme in reality. Thus, the need to develop a simulator for ATM networks has been felt. Discrete-event simulation technique offers the greatest degree of flexibility in modeling a simulator for an ATM network. Due to this reason ATMSIM, the simulator for ATM networks, is developed using discrete-event simulation technique. ATMSIM allows the user to define a network configuration consisting of a physical network and specifications of the components of the network. ATMSIM provides both steady-state and instantaneous performance measures to facilitate the study of complex dynamics that arises in ATM networks. ATMSIM has been used as a testbed for emulating a virtual reality model for ATM networks and to study the above mentioned schemes.